Preheat oven to 325 degrees F (165 degrees C). Grease a baking sheet.
Melt butter in a large skillet over medium heat; cook and stir mushrooms in butter until softened, about 5 minutes. Transfer mushrooms to a plate and allow to cool.
Heat another skillet over medium-high heat; cook and stir sausage, onion, and garlic until meat is crumbly, evenly browned, and no longer pink, about 10 minutes. Drain and discard any excess grease. Transfer mixture to a large bowl and stir in bread crumbs, Parmesan cheese, mozzarella cheese, Italian seasoning, and parsley.
Arrange mushrooms gill sides up on prepared baking sheet. Spoon sausage mixture into mushroom caps.
Bake in preheated oven until cheese is melted, about 30 minutes.
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