Start by cleaning and slicing about 8 ounces of fresh mushrooms and 1 medium onion. Aim for thin, even slices to ensure uniform cooking. Set aside the prepared vegetables on a cutting board.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the sliced onions first and sauté for 4-5 minutes until they become translucent. Then, stir in the garlic and mushrooms, cooking for an additional 5-7 minutes until they soften.

In a mixing bowl, combine the sautéed mushrooms and onions with 2 cups of shredded cheese, along with salt, pepper, and any herbs you want to add. Stir until everything is evenly coated.

Preheat your oven to 350°F (175°C). Transfer the mixture into a greased baking dish, spreading it evenly. For an extra cheesy top, sprinkle an additional ½ cup of cheese on top. Bake uncovered for about 30 minutes until bubbly and golden brown.

Once baked, remove the casserole from the oven and allow it to cool for about 5-10 minutes before serving. This lets the dish firm up slightly, making it easier to slice.