Name: Creamy Mushroom and Green Bean Casserole with Crispy Baked Onions Recipe
Prep time: 20 min
Cook time: 1 h
Total time: 1 h 20 min
Cuisine: American
Ingredients: 1½ pounds green beans (trimmed and halved); 16 ounces cremini mushrooms (sliced); 4 garlic cloves (minced); 2 tablespoons fresh thyme leaves; 2 tablespoons extra-virgin olive oil; 2 tablespoons tamari; ¼ cup all-purpose flour; 3 cups milk (any kind); 1½ teaspoons Dijon mustard; ½ cup grated Parmesan cheese (plus more for sprinkling); ¼ teaspoon freshly ground black pepper; 1½ cups French fried onions (or homemade Crispy Baked Onions)

If you’re on the hunt for a side dish that’s both comforting and a little fancy, you’ve got to try this Creamy Mushroom and Green Bean Casserole with Crispy Baked Onions Recipe. It’s like the classic green bean casserole got a delicious upgrade with fresh mushrooms and homemade creamy sauce, finished off with irresistibly crispy baked onions. Whether you’re bringing it to a holiday dinner or just craving some cozy veggies, this recipe never disappoints.

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Why You’ll Love This Recipe

Fresh ingredients, better flavor: Using fresh green beans and cremini mushrooms makes the sauce richer and the texture delightfully crisp.
Creamy, comforting sauce: I’ve perfected a silky mushroom sauce that’s thick but not heavy—trust me, it’s addictive.
Crispy baked onions on top: Homemade crispy onions take this casserole from everyday to wow with a satisfying crunch.
Great make-ahead dish: You can prep most of it before guests arrive and simply bake before serving.

A black rectangular baking dish filled with three layers: the bottom layer is bright green fresh green beans arranged neatly around the edges; the middle layer is a creamy beige mushroom sauce spread over the green beans with visible sliced mushrooms; the top layer consists of golden crispy fried onions being placed by a woman's hands, which are holding a handful of the onions above the dish. Next to the baking dish, on the right, there is a small wooden plate with more crispy fried onions on a white marbled surface background. Photo taken with an iphone --ar 4:5 --v 7

Ingredients You’ll Need

The real magic here happens when fresh green beans meet a luscious mushroom sauce with just the right punch of garlic and thyme. These ingredients work beautifully together to give you that classic comfort flavor but fresher and more vibrant.

Green beans: Fresh and trimmed, they provide a perfect crisp-tender texture after blanching.
Extra-virgin olive oil: Adds a rich, fruity base to cook the mushrooms without overpowering.
Cremini mushrooms: Their earthy depth brings serious umami to the sauce.
Tamari: A gluten-free soy sauce alternative that boosts flavor while keeping saltiness balanced.
Garlic cloves: Fresh minced garlic amps up the savory notes in every bite.
Fresh thyme leaves: Sometimes forgotten but essential for that subtle herbal brightness.
All-purpose flour: For thickening our creamy mushroom sauce to just the right consistency.
Milk: I use whole milk for creaminess, but feel free to swap anything you prefer here.
Dijon mustard: Just a touch to elevate the sauce with a little tanginess.
Parmesan cheese: Adds nutty richness inside the sauce and a sprinkle on top.
Freshly ground black pepper: To taste, because seasoning is everything.
French fried onions or homemade crispy baked onions: The star crunchy topping that your family will go crazy for.

Note: You’ll find the complete list of ingredients, along with their exact measurements, in the printable recipe card at the bottom of this post.

Variations

I love how flexible this casserole is. Over the years, I’ve tweaked it a bit to suit different moods, seasons, and dietary needs, so feel free to make your own spin on it!

Use fresh herbs: Sometimes I swap thyme for rosemary or sage, which adds a slightly different but equally delicious flavor.
Make it vegan: Substitute the milk for almond or oat milk, use a dairy-free Parmesan alternative, and replace butter with olive oil – it still turns out beautifully creamy.
Add toasted nuts: Sprinkling toasted almonds or pecans on top of the onions gives an unexpected crunch and nuttiness.
Switch up the mushrooms: Try portobello or shiitake mushrooms for a meatier texture and deeper flavor.

How to Make Creamy Mushroom and Green Bean Casserole with Crispy Baked Onions Recipe

Step 1: Blanch Your Green Beans to Perfection

First, bring a large pot of salted water to a rolling boil. This helps season the green beans as they cook. Drop in your trimmed green beans and blanch them for exactly 4 minutes—this keeps them bright green and crunchy-tender. Then, immediately plunge them into an ice-water bath to stop the cooking and preserve that vibrant color. Pat them dry gently on a towel afterward. This step is critical so your beans don’t get mushy in the casserole.

Step 2: Sauté Mushrooms with Garlic and Herbs

Next, heat olive oil in a large skillet over medium heat. Add your sliced cremini mushrooms and cook until they’re soft and their moisture evaporates—around 8 to 10 minutes. Stir in tamari, minced garlic, and fresh thyme leaves for that lovely layered flavor. You’ll want to smell that heavenly aroma filling your kitchen—it’s the moment you know this dish is coming together beautifully.

Step 3: Make the Creamy Mushroom Sauce

Sprinkle the flour evenly over the mushrooms and mix well to coat everything—this helps thicken your sauce. Slowly pour in the milk while continuously stirring or whisking to avoid lumps. Add Dijon mustard for a subtle kick. Let the sauce simmer gently for 15 to 20 minutes until it thickens into a velvety texture. Finish it off by stirring in Parmesan cheese and freshly ground pepper. Taste and adjust seasoning if you need to; this sauce is what ties the whole casserole together.

Step 4: Assemble and Bake Your Casserole

Spread one-third of your mushroom sauce in the bottom of a greased 9×13-inch baking dish. Layer your blanched green beans over the sauce, then pour the remaining sauce evenly on top. Now comes the fun part — generously sprinkle your crispy baked onions (or store-bought French fried onions if you’re short on time) all over the top.

Cover the dish and bake at 400°F for 10 minutes. Then uncover and bake for another 10 minutes to get the onions beautifully golden and crispy. If you notice they’re browning too quickly, just cover the casserole loosely with foil—you want that perfect crunch without any burnt spots!

When it’s out of the oven, I sprinkle a little more Parmesan on top—because why not?

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Pro Tips for Making Creamy Mushroom and Green Bean Casserole with Crispy Baked Onions Recipe

Don’t skip the ice bath: It’s the trick that keeps your green beans perfectly crisp and vibrant, avoiding dull, limp veggies.
Whisk constantly when adding milk: This prevents lumps and gives you that dreamy sauce texture every time.
Make crispy onions at home: I learned making baked onions is easier than I thought and tastes so much better than store-bought.
Cover if onions brown too fast: It’s an easy way to keep the topping golden without burning while the casserole finishes baking.

How to Serve Creamy Mushroom and Green Bean Casserole with Crispy Baked Onions Recipe

A rectangular metal baking dish holds a green bean casserole with three distinct layers: the bottom layer is bright green cooked green beans, the middle layer is a creamy light beige mushroom sauce with visible small mushroom slices scattered throughout, and the top layer consists of golden-brown crispy fried onion strips piled thickly, some slightly curled with a crunchy texture. The dish is set on a white marbled surface with a gray and white cloth nearby. Photo taken with an iphone --ar 4:5 --v 7

Garnishes

I usually skip complicated garnishes here because the crispy onions and Parmesan do all the work. But adding a sprinkle of fresh thyme or chopped parsley right before serving adds a lovely pop of color and fresh herbal flavor.

Side Dishes

This casserole pairs beautifully with roasted turkey or chicken and creamy mashed potatoes. I also love serving it alongside a crisp green salad to balance all the richness and make the meal feel special.

Creative Ways to Present

For holiday dinners, I sometimes divide the casserole into individual ramekins topped with a delicate sprig of thyme. It always impresses guests and makes serving easier. Another fun idea is to layer the casserole in clear glass baking dishes so the beautiful layers are visible—definitely a showstopper!

Make Ahead and Storage

Storing Leftovers

After your meal, store leftovers in an airtight container in the fridge for up to 3 days. I’ve found that the casserole actually tastes even better the next day once the flavors meld together. Just keep the crispy onions separate if you want to maintain that crunch when reheating.

Freezing

I’ve successfully frozen this casserole before baking it—just assemble everything except the onions, cover tightly with foil, and freeze for up to 3 months. Take it out the day before and thaw in the fridge. Add fresh or store-bought onions right before baking for best results.

Reheating

To reheat without losing that lovely texture, warm your leftovers in a 350°F oven until heated through, about 15-20 minutes. If you stored onions separately, sprinkle them on top and give it an additional 5 minutes to crisp back up. Avoid the microwave if you can—it tends to make the green beans mushy and onions soggy.

FAQs

Can I use frozen green beans for this casserole?

Absolutely! If you’re using frozen green beans, thaw and drain them well before assembling the casserole. You might skip the blanching step since frozen beans are usually pre-cooked, but keep an eye on the texture to avoid overcooking during baking.
How do I make crispy baked onions for the topping?

To make crispy baked onions, thinly slice onions, toss them with a little oil, salt, and flour, then spread on a baking sheet and bake at 400°F, stirring occasionally, until golden and crispy—usually about 20-25 minutes. It’s well worth the extra step for that crunchy, fresh taste.
Can I make this casserole ahead of time?

Yes! You can prepare the green beans and mushroom sauce a day ahead. Assemble the casserole right before baking to keep everything fresh. Alternatively, assemble the whole dish minus the onions and refrigerate overnight, then add the onions and bake when ready to serve.
Is there a way to make this gluten-free?

Definitely. Swap the all-purpose flour for a gluten-free flour blend or cornstarch for thickening the sauce. Just be sure to check your tamari or soy sauce is gluten-free. Use gluten-free fried onions or homemade crispy onions made with gluten-free flour.

Final Thoughts

This Creamy Mushroom and Green Bean Casserole with Crispy Baked Onions Recipe has become one of my go-to side dishes whenever I want something comforting yet fresh. It’s simple enough to make on a weeknight but special enough for holidays. I love sharing it with friends because they always ask for the recipe afterward. I know you’ll enjoy it too — it’s the perfect balance of creamy, crispy, and full of flavor. Give it a try for your next meal and watch your family go crazy for it!