Preheat oven to 350° F. Wash mushrooms, remove stems and dice into small pieces. Dice cnion. Saute mushroom stems and onion in 1/2 of butter until golden. Fielt other 1/2 stick butter and mix with enough bread crumbs to moisten. Add stems and onion. Mix Parmesan cheese, salt and white pepper. Stuff into mushroom caps and bake 325° for 35 minutes or until tender. Serve hot.