In a large skillet over medium heat, saute the onion in butter until tender. Transfer to a large bowl. Stir in the next eight ingredients. Divide between 2 ungreased 13 inchx9 inchx2 inch baking pans. Stir 2 cups of water into each pan. Cover and bake at 180ºC or 1.5 hours. Uncover; sprinkle with cheese. Return to the oven for 5 to 10 minutes or until cheese melts.