Preheat oven to 400o F.
Lightly spray 9" X 13" X 2" baking dish.
Sprinkle choice of filling, onion, cheese and optional additions in prepared baking dish.
Put eggs, milk, Bisquick Mix and pepper in blender; blend about 15 seconds on high. (May make 2 batches or use mixer)
Pour into baking dish over ingredients.
Bake at 400o F for 50 - 70 minutes until top is golden brown and knife inserted in center comes out clean.
Cool 5 minutes before cutting into squares.
Serves 12 - 15.