In a skillet, saute the mushrooms and onions in 1 tablespoon butter until tender. In a large bowl, combine the flour, baking powder and salt. , In another bowl, combine eggs and cheeses. Melt remaining butter; add to egg mixture. Stir into dry ingredients along with mushroom mixture., Fill greased muffin cups three-fourths full. Bake at 350° for 35-40 minutes or until a knife inserted in the center comes out clean. Carefully run the knife around edge of muffin cups before removing.