Brown the following:

1 lb. hamburger
1/2 cup onion (chopped)

Stir in the following and then put mixture into a greased casserole:

1 envelope dry Taco seasoning

Beat together and pour over the meat in the casserole:

1 1/4 cups milk
3/4 cup Bisquick
3 eggs

Bake uncovered at 375ºF for 30 minutes.  Top with half of the following:

2 tomatoes
1 cup grated cheddar cheese

Bake an additional 8-10 minutes, then top with:

the remaining tomato and cheese
torn lettuce to cover