Deliciously cheesy mushroom casserole is wonderfully flavored with onions, thyme, and melted gruyere. Made without breadcrumbs, it's keto and gluten-free.
This casserole is amazing. It's so cheesy and flavorful! Just 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 make them.
The ingredients you'll need
You'll only need a few wholesome ingredients to make this tasty mushroom casserole (the exact measurements are included in the recipe card below):
Olive oil: A delicious oil. But if you're unsure about cooking with it, 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. I'm lazy, so I use the food processor to chop onions, and I use minced garlic from a jar.
Spices: Dried thyme and parsley. Make sure they are fresh! A stale spice can easily ruin a dish.
Shredded Gruyere cheese. Gruyere tastes best, but I've made this casserole with shredded sharp cheddar and it was good too.
How to make a mushroom casserole
Since mushrooms release quite a bit of liquid when cooked, you can't just put them in a dish and bake. You need to saute them first.
So to make this dish, 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 with cheese and bake until the cheese is melted.
If your casserole dish is broiler-safe, you can also simply broil the casserole for 2-3 minutes, in the middle of the oven. You don't want to get too close to the heating element and risk burning the casserole.
What mushrooms to use? What cheese?
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!
As for the cheese, 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.
How to serve mushroom casserole
They are also excellent with chicken. Sometimes I slice baked chicken breast and top the slices with this casserole.
How to keep the 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 on 50% power.
More tasty mushroom recipes
I have so many mushroom recipes on this blog! It's difficult to choose. But a couple of my absolute favorites are this mushroom frittata which is perfect for brunch, and these delicious baked shiitake mushrooms.
I typically publish a new recipe once a week. Want the new recipes in your inbox? Subscribe!
- ¼ cup olive oil, plus more for the pan
- 32 oz sliced baby Bella or crimini mushrooms
- 2 teaspoons Diamond Crystal kosher salt
- 1 cups chopped onions
- 1 tablespoon minced fresh garlic
- ½ teaspoon black pepper
- ½ teaspoon dried thyme
- 1 cup shredded gruyere cheese
- 1 teaspoon dried parsley for garnish
- 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, the onion, and the 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, one more minute. Remove from heat.
- Transfer the mixture to the prepared baking dish. Sprinkle with the shredded cheese.
- Bake until hot and bubbly, about 15 minutes. Sprinkle with parsley and serve.