Melt butter in a small frying pan over medium heat; sauté garlic and onions until tender.
Add sausage to pan; brown and break up; discard any excess fat.
Combine all ingredients in a mixing bowl.
Preheat the oven to 400°F; spoon filling into the well of each mushroom; place on a broiler pan or a lined baking sheet; sprinkle with Parmesan cheese.
Bake ~15 minutes, until tops begin to brown.