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, salt and pepper till smooth 15 seconds in blender on High or 1 minute with hand beater. Pour into plate. Bake till knife inserted in centre comes out clean, 25 to 30 minutes. Top with tomato slices and remaining cheese is melted 3 to 5 minutes. Cool 5 minutes.