1. Prepare the Mushrooms: In a large skillet, melt 1 tablespoon of butter over medium heat. Add the chopped onions and sauté until translucent, about 5 minutes. Toss in the minced garlic and cook for another minute until fragrant. Then, add the sliced mushrooms, season with salt and pepper, and cook until the mushrooms have softened and any released moisture has evaporated, about 7–8 minutes. Remove from heat and let the mixture cool slightly.

2. Prepare the Cheese Mixture: In a large bowl, combine the cream cheese and sour cream. Add the grated cheddar and mozzarella cheeses, and season with salt and pepper to taste. Mix until well combined.

3. Assemble the Casserole: Preheat your oven to 350°F. Grease a baking dish with the remaining tablespoon of butter. Spread the sautéed mushroom mixture evenly in the bottom of the dish. Dollop the cheese mixture over the mushrooms and spread it out evenly. Sprinkle the breadcrumbs on top.

4. Bake: Place the casserole in the preheated oven and bake for 25–30 minutes, or until the top is golden brown and the cheese is bubbly.

5. Serve: Allow the casserole to cool for a few minutes before serving and enjoy the melty, cheesy goodness!