Cheesy Mushroom Casserole

This cheesy mushroom casserole is my go to when I want something hearty without too much fuss. It comes together quickly using simple ingredients yet fills the kitchen with an irresistible aroma. Perfect as a side or a vegetarian main, this dish feels like comfort food made easy.

I threw this together on a busy weeknight and it instantly became a crowd favorite at home. The combination of mushrooms and cheese creates a satisfying depth that keeps me coming back.

Ingredients

• Garlic: Brings a robust, aromatic punch. Freshly minced is best to keep its bright flavor.
• Baby Bella or crimini mushrooms: Provide a meaty texture and deep earthiness. Look for firm, dry caps without dark spots.
• Gruyere cheese: Melts beautifully giving a nutty richness to the casserole. Buy a quality block and shred fresh whenever possible.
• Dried thyme: Adds a subtle herbal note that complements the mushrooms.
• Chopped onions: Build sweetness and complexity in the base. Fresh and crisp ones create the best results.
• Dried parsley: Is for garnish, offering a fresh color contrast and mild bitterness.
• Kosher salt: Is preferred to control seasoning precisely. Diamond Crystal is my go to for its balance.
• Black pepper: Adds a mild heat that ties the flavors together.
• Olive oil: Is used for sautéing and greasing the dish, opt for extra virgin for the best flavor.

Instructions

Preheat and Prepare:

Preheat your oven to 400 degrees Fahrenheit. Oil a 2-quart baking dish well so the casserole doesn't stick and browns nicely on the edges.

Sauté Mushrooms and Onions:

Warm the olive oil in a large deep skillet over medium-high heat. Add the mushrooms, onions, and kosher salt. Cook them stirring occasionally until the mushrooms release their moisture and the liquid mostly evaporates. This takes about 10 minutes and is critical to prevent the dish being soggy while concentrating flavors.

Add Garlic and Herbs:

Stir in the minced garlic, black pepper, and thyme. Continue cooking for one more minute to bloom the spices and soften the garlic without burning it. Then remove the pan from heat to preserve those fresh aromas.

Assemble the Casserole:

Transfer the cooked mushroom mixture to the prepared baking dish and evenly sprinkle the shredded Gruyere cheese over the top. The cheese will melt and create a golden crust as it bakes.

Bake and Garnish:

Place the dish in the heated oven and bake until hot, bubbly, and the cheese has melted perfectly, about 15 minutes. Once out of the oven, sprinkle with dried parsley for a burst of color and light herbal flavor. Serve immediately.

Gruyere is my favorite cheese for this because it melts evenly and adds a sweet, slightly nutty flavor that brings out the best in the mushrooms. I remember my first attempt when I skipped adding enough salt to the mushrooms and ended up with a flat dish — seasoning at the sauté stage is key to a flavorful casserole.

Storage tips

Store any leftovers in an airtight container in the refrigerator for up to three days. To reheat, cover with foil and warm in a 350-degree oven to keep the cheese smooth and avoid drying out the mushrooms.

Ingredient swaps

You can swap Gruyere for other melting cheeses like Fontina or Monterey Jack if you prefer a milder taste. Use cremini mushrooms if baby Bellas are not available or mix in shiitake for extra umami. Fresh thyme can replace dried if you have it on hand but add it at the end of cooking to preserve brightness.

Serving ideas

This casserole pairs well with roasted meats, a fresh green salad, or simply crusty bread to soak up the juices. For a vegetarian meal, serve alongside quinoa or whole-grain rice for a complete protein boost.

Seasonal twists

Add chopped spinach or kale in the last few minutes of sautéing for a vitamin boost and color contrast. In colder months, stirring in a pinch of smoked paprika can add a lovely warmth and depth. For spring, fresh herbs like parsley and chives sprinkled on top brighten the dish instantly.

This warm and hearty casserole delivers comfort in every bite and can easily become a staple in your meal rotation.

Frequently Asked Recipe Questions

→ What type of mushrooms work best?

Baby Bella or cremini mushrooms provide an earthy flavor and firm texture that holds up well when cooked and baked.

→ Can I use another cheese instead of Gruyere?

Yes, cheeses like Swiss, mozzarella, or fontina can be substituted for a similar melt and mild nutty flavor.

→ How do I prevent the casserole from becoming soggy?

Cooking the mushrooms and onions until most liquid evaporates ensures the casserole stays firm and not watery.

→ Can I add other vegetables?

Absolutely, spinach, bell peppers, or zucchini can complement the mushrooms and add extra color and nutrients.

→ What is the best way to reheat leftovers?

Reheat gently in a 350°F oven until warmed through, preserving the texture and flavor.