Prepare the Mushrooms: Preheat your oven to 375°F (190°C). Heat olive oil in a skillet over medium heat. Sauté sliced Portabella mushrooms with garlic until softened, about 5 minutes. Season with salt, pepper, and Italian seasoning.
Assemble the Casserole: Lightly grease a baking dish. Spread the cooked mushrooms evenly in the dish. Pour heavy cream over the mushrooms and sprinkle the mozzarella cheese on top.
Optional Crunchy Topping: If using breadcrumbs, sprinkle them over the cheese for a golden crust.
Bake: Place the casserole in the preheated oven and bake for 20-25 minutes, or until the cheese is melted and bubbly.
Garnish and Serve: Remove from the oven, let it rest for 5 minutes, and garnish with fresh parsley. Serve warm and enjoy!