This deliciously cheesy mushroom casserole is wonderfully flavored with onions, thyme, and melted gruyere.
Made without breadcrumbs and ready in about 40 minutes, it's keto and gluten-free.
This casserole is amazing. It's so cheesy and flavorful! The wonderful aroma it releases when you cook it will let you know how good it's going to be.
Mushrooms are one of my favorite vegetables (well, technically they're fungi, not vegetables). And this tasty casserole is one of my favorite ways to cook them.
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Ingredients
You'll only need a few simple ingredients to make this mushroom casserole. The exact measurements are included in the recipe card below. Here's an overview of what you'll need:
Olive oil: A delicious oil and one of my favorites to cook with. It does have a relatively low smoke point, so you can use avocado oil instead.
Baby Bella mushrooms: You can also use white mushrooms if that's all you can find.
Salt and pepper: I use kosher salt in most of my recipes. If using fine salt, you will need to reduce the amount you use.
Aromatics: Onions and garlic. While jarred minced garlic is convenient, freshly minced tastes significantly better.
Spices: I use dried thyme and parsley.
Shredded Gruyere cheese: Gruyere tastes best, but I've made this casserole with shredded sharp cheddar and it was good too.
Instructions
Since mushrooms release quite a bit of liquid when cooked, you can't just put them in a dish and bake them. You need to saute them first.
So to make this recipe, I saute the mushrooms and chopped onions in olive oil until the mushrooms have released their liquid and most of the liquid has evaporated.
Check out the video below in the recipe card to see the process. See the steam? That's the mushroom's liquid, evaporating.
Once the liquid has mostly evaporated, you can transfer the mushrooms and onions to a greased baking dish, top them with cheese, and bake the casserole until the cheese has melted.
Expert tip
If your casserole dish is broiler-safe, you can simply broil the casserole for 2-3 minutes, in the middle of the oven, until the cheese is melted. You don't want to get too close to the heating element and risk burning the casserole.
Frequently asked questions
I like to use brown mushrooms in this recipe because it makes the casserole look more presentable than white mushrooms. So I use baby Bella or crimini mushrooms.
I use quite a lot of mushrooms - 32 oz. But keep in mind that mushrooms lose significant volume after they are cooked. Once cooked, they fit nicely in a 2-quart casserole dish.
Since I'm lazy, I buy pre-sliced mushrooms at Whole Foods. I also buy pre-chopped onions. Which makes this casserole super easy!
Gruyere goes wonderfully with mushrooms. But sometimes, when I have an open bag of pre-shredded Italian cheeses in the fridge, I use that instead.
Another tasty option is smoked gouda. It adds a wonderfully smoky flavor to the casserole.
No, unfortunately, that won't work. As explained above, mushrooms release quite a bit of liquid when cooked, so you can't just put them in a dish and bake them. You need to saute them first and get rid of some of that liquid.
Variations
A good way to vary this recipe is by using different cheeses, such as cheddar, gouda, or provolone.
Another way is to experiment with different herbs and spices. Good options include smoked paprika, ground cumin, and dried oregano.
Serving suggestions
Mushrooms go well with steak, so I often make this recipe when I serve reverse-seared steak or London broil.
They are also excellent with chicken. Sometimes I slice baked chicken breast and top the slices with this casserole.
Since the mushrooms are not very colorful, it's nice to have a colorful veggie as a side, such as steamed broccoli or tomato salad.
Storing leftovers
Once the leftovers are cool, transfer them with a slotted spoon to an airtight container. Keep them in the fridge for up to 4 days. Reheat them in the microwave at 50% power.
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Recipe Card
Cheesy Mushroom Casserole
Ingredients
- ¼ cup olive oil - plus more for the pan
- 32 ounces baby Bella or crimini mushrooms - sliced
- 2 teaspoons Diamond Crystal kosher salt - or 1 teaspoon of any other salt
- 1 cup onions - chopped
- 1 tablespoon fresh garlic - minced
- ½ teaspoon black pepper
- ½ teaspoon dried thyme
- 1 cup Gruyere cheese - shredded
- 1 teaspoon dried parsley - for garnish
Instructions
- Preheat your oven to 400 degrees F. Grease a 2-quart baking dish with olive oil.
- Heat the olive oil in a large, deep skillet over medium-high heat. Add the mushrooms, onions, and kosher salt. Cook, stirring occasionally, until the mushrooms have released their liquid and the liquids have mostly evaporated, about 10 minutes.
- Add the garlic, black pepper, and thyme. Cook, stirring, for one more minute. Remove from heat.
- Transfer the mixture to the prepared baking dish. Sprinkle it with the shredded cheese.
- Bake the casserole until hot and bubbly, about 15 minutes. Sprinkle with parsley and serve.
Video
Notes
- If your casserole dish is broiler-safe, instead of baking, you can simply broil the casserole for 2-3 minutes in the middle of the oven until the cheese is melted. You don't want to get too close to the heating element and risk burning the casserole.
- Once the leftovers are cool, transfer them with a slotted spoon to an airtight container. Keep them in the fridge for up to 4 days. Reheat them in the microwave at 50% power.
Nutrition per Serving
Disclaimers
Cup measurements refer to the standard American cup, which is 240 milliliters. Most of my recipes are low-carb (or keto) and gluten-free, but some are not. Please verify that a recipe fits your needs before using it. Recommended and linked products are not guaranteed to be gluten-free. Nutrition info is approximate, and the carb count excludes non-nutritive sweeteners. Nutrition info may contain errors, so please verify it independently. Recipes may contain errors, so please use your common sense when following them. Please read these Terms of Use carefully before using any of my recipes.
Vered DeLeeuw, LL.M., CNC, has been following a low-carb real-food diet and blogging about it since 2011. She's a Certified Nutrition Coach (NASM-CNC), has taken courses at the Harvard School of Public Health, and has earned a Nutrition and Healthy Living Certificate from Cornell University. Her work has appeared in several major media outlets, including Healthline, HuffPost, Today, Women's Health, Shape, and Country Living. 
Donna D. Wolfe
I made this for Thanksgiving dinner. It was a very tasty with turkey. I will definitely be making this again. Thank you for sharing.
Vered DeLeeuw
You're very welcome, Donna! 🙂
Becky
Followed recipe exactly. Was amazing! So flavorful! Making again this evening! Yum!
Vered DeLeeuw
Yay! I'm so glad you liked this recipe, Becky! Thank you for the comment.
Dianna
I made this recipe last night for dinner, using Shitake mushrooms and mozzarella cheese. Oh my goodness! They were so good and my hubby loved them! We grow our own Shitake mushrooms, which is why I used those.
Vered DeLeeuw
I'm so glad you enjoyed this casserole, Dianna! I love shiitake mushrooms. How lucky you are to be growing them!
Sara
It was so good that my 80 yr old Dad asked for the leftovers and recipe!
Vered DeLeeuw
Yay! I'm so glad this was a success, Sara! Thanks for the review.
Lindsay
I made this for Thanksgiving and it turn out just perfect!
Vered DeLeeuw
Wonderful, Lindsay! I'm so glad you enjoyed this casserole.
Lynn
Who would have thought to combine cheese with mushrooms?!! But this works! We all enjoyed this as one of our sides for dinner guests; it was such a nice addition to other vegetables, in a non-competitive way. I used sliced portobello mushrooms which also gave a lovely flavour. Thank you..who would have thought?! 🙂
Vered DeLeeuw
I'm so glad you liked this recipe, Lynn! I love the idea of using portobello mushrooms.
Leslie
Oh, Sooo Good!!
Vered DeLeeuw
Glad you liked it, Leslie!
Lynn
Our love of mushrooms knows no bounds, so when we saw this recipe, we knew we had to try it at our pre-Christmas gathering and it was a hit! I didn't change a thing, assembled it early in the day and because oven space was at a premium, simply nuked it to melt the cheese and reheat the mushrooms; as a make-ahead dish...full marks! Thank you for publishing this yummy recipe!
Vered DeLeeuw
I'm so glad this recipe was a success, Lynn! It's interesting to learn that you can microwave it to melt the cheese. Thank you for this tip.
Patty
I made this and it was a hit with my family. I’m going out of town and would like to bring this as a side dish. Can I make this, put it in a baking dish, refrigerate it and then bake it the next day? Thanks!
Vered DeLeeuw
Yes, absolutely. If I were you, I would pre-cook the mushroom/onion mixture (steps 2,3) and put it in a food storage container. Bring the shredded cheese in a separate bag or container. When ready to bake the casserole, grease the baking dish, assemble the casserole (drain any liquids from the mushroom/onion mixture before adding it to the baking dish), and bake it.
Lija
I'm a mushroom lover, so I can't wait to try this recipe. I liked your tip to broil rather than bake the casserole for 2-3 minutes until the cheese is melted. I was thinking I could actually use my air fryer that's like a toaster oven instead of lighting the big oven.
Also, since I always stir-fry mushrooms and onions when we have steak, I am looking forward to using this recipe when the husband grills steaks again. Thanks for the advice!
Vered DeLeeuw
You're very welcome, Lija! I hope you enjoy it.