In a small skillet, saute mushrooms and green pepper in butter until tender. In a large bowl, combine the eggs, flour, baking powder and salt if desired. Add mushroom mixture. Stir in remaining ingredients.
Pour into a greased 13x9-in. baking dish. Bake, uncovered, at 400° for 15 minutes. Reduce heat to 350°; bake 25-35 minutes longer or until a knife inserted in the center comes out clean.