Preheat oven to 375°F (190°C) with rack in the top third position and grease an 8x8-inch baking dish with cooking spray.

Melt butter in a large skillet over medium-high heat. Add breadcrumbs and cook until fragrant, about 2 minutes. Transfer to a bowl and set aside. Clean the skillet.

Cook sausage in the skillet over medium-high heat, breaking into bite-size pieces, for 2-3 minutes until browned.

Add mushrooms, garlic, black pepper, and salt to the skillet and cook until soft and excess moisture evaporates, 12-15 minutes.

Stir sour cream, cream cheese, parsley, mozzarella cheese (1/4 cup), and Parmesan (1/4 cup) into the mushroom mixture until smooth and combined.

Transfer mixture to the prepared baking dish, spread evenly, and sprinkle the remaining mozzarella (1/2 cup), Parmesan (1/4 cup), and breadcrumbs on top.

Bake for 12 minutes until cheese is melted and bubbling. Broil for 1-2 minutes until breadcrumbs are browned.