Preparation / Directions:
Preheat oven to 400F Grease pie plate 10x1 1/2".
Cook onions and butter in 10" skillet over low heat, stirring occasionally, until onions are a light brown. Sprinkle tuna, 1 cup of cheese and onions in plate.
Beat eggs, milk, Bisquick, salt and pepper until smooth 15 seconds in blender on High or 1 minute with hand beater. Pour into plate.
Bake until knife inserted in center comes out clean, 25 to 30 minutes. top with tomato slices and remaining cheese is melted 3 to 5 minutes. Cool 5 minutes.
Source: "The Best of Bisquick"