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.
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.
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 casserole, 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 is melted.
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. You need to saute them first and get rid of some of that liquid.
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.
They are also excellent with chicken. Sometimes I slice baked chicken breast and top the slices with this 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 on 50% power.
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Cheesy Mushroom Casserole
- ¼ cup olive oil plus more for the pan
- 32 oz baby Bella or crimini mushrooms sliced
- 2 teaspoons Diamond Crystal kosher salt or 1 teaspoon of fine 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
- 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.