In a large skillet, saute the mushrooms, onion and garlic in butter until tender. Add the green pepper; saute 1 minute longer. Remove from the heat; drain. In a large bowl, beat eggs. Stir in the cheeses, flour, baking powder, salt, basil and nutmeg. Add mushroom mixture. Pour into a greased 15-in. x 10-in. x 1-in. baking pan.
Bake, uncovered, at 350 degrees F for 30-35 minutes or until edges are golden and a knife inserted near the center comes out clean. Let stand for 15 minutes. Cut into squares; serve warm.