Impossible Tuna and Cheddar Pie

2 cups chopped onions
1/4 cup butter or margarine
2 (6 1/2 ounce) cans solid white tuna in water, drained
2 cups shredded Cheddar cheese, divided
3 eggs
1/4 cup milk
1 cup Bisquick
1/8 teaspoon pepper
2 tomatoes, thinly sliced

Heat oven to 400 degrees F. Grease a casserole dish or glass pie plate.

Cook onions and butter over low heat until light brown. Sprinkle tuna and 1 cup of cheese into baking dish.

Beat eggs, milk, Bisquick and pepper in blender on high for 15 seconds. Pour into baking dish. Bake for 25 to 30 minutes or until a knife inserted in center comes out clean. Top with remaining cheese and the tomatoes. Bake 3 to 5 minutes longer until cheese is melted.