Preheat oven to 400 degrees.
Clean mushrooms. remove stems and set aside.
Brush mushroom caps with part of butter and sprinkle them with garlic salt.
Chop stems and saute with onion in remaining butter. Add other ingredients and fill caps with this mixture. These can be done ahead to this point and refrigerated.
Place in an oven-proof dish and bake about 10 minutes, just until mushrooms are tender and tops are browned.