Cheesy Mushroom Casserole

Name: Cheesy Mushroom Casserole
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Ingredients: 1/4 cup olive oil (plus more for the pan); 32 ounces baby Bella or crimini mushrooms (sliced); 2 teaspoons Diamond Crystal kosher salt; 1 cup onions (chopped ); 1 tablespoon fresh garlic (minced ); 1/2 teaspoon black pepper; 1/2 teaspoon dried thyme; 1 cup Gruyere cheese (shredded ); 1 teaspoon dried parsley (for garnish)

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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.

Mushroom casserole is served in a white baking dish with a napkin.

This casserole is amazing. It's so cheesy and flavorful! The wonderful aroma it releases when you cook it will be your first hint that it's going to be sooo good. Mushrooms are one of my favorite vegetables (well, technically, they're fungi, not vegetables), and this delicious casserole is one of my favorite ways to cook them (these roasted mushrooms are amazing, too).

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.
Salt and pepper: I use kosher salt and freshly ground black pepper.
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.

Variations

A good way to vary this recipe is 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. You can add a generous pinch of any additional spice.

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.

Two photos showing the process for cooking mushrooms.

Once the liquid has mostly evaporated, you can transfer the mushrooms and onions to a greased baking dish and top them with cheese.

The last step is baking the casserole until the cheese has melted.

Expert Tip

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.

Recipe FAQs

What mushroom variety should I use?

I like to use brown mushrooms in this recipe because they make the casserole look more presentable than white mushrooms. So, I typically use baby Bella or crimini mushrooms.

I use quite a lot of mushrooms - 32 ounces. 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!

What cheese is best in this casserole?

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.

Can I skip sauteing the mushrooms and simply bake them?

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 remove some of that liquid.

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 and keep them in the fridge for up to 4 days. Reheat them in the microwave at 50% power.