Preheat your oven to 350 F. Combine sausage, onion, sage, garlic salt & black pepper in a skillet over medium heat.
Mix and cook until sausage is browned. Remove from heat and drain grease if necessary.
Add the cheese and mix to allow the cheese to bind the sausage together.
Spoon sausage and cheese mix into the mushroom caps and put the caps into a baking dish. Bake for 15 minutes and serve hot!