In a saucepan, saute onion in butter until tender. Stir in chicken, chilies and mushrooms. In a small bowl, combine soup, sour cream and hot pepper sauce; add to pan. Cook and stir for 5 minutes. Sprinkle mozzarella cheese in the bottom of pie shell; add chicken mixture. Sprinkle with cheddar cheese. Bake at 350° for 25-30 minutes or until filling is bubbly and cheese is melted.