IN A LARGE BOWL, COMBINE THE CORN BREAD, EGG AND MILK UNTIL BLENDED. SPREAD INTO AN 8 X 8 PAN SPRAYED WITH COOKING SPRAY. BAKE AT 350 FOR 15 MINUTES. COMBINE TACO MEAT AND BEANS, SPREAD OVER CORN BREAD. COMBINE SOUR CREAM, 3/4 CUP CHEESE AND ONION. SPREAD OVER MEAT MIXTURE. BAKE FOR 20 – 25 MINUTES LONGER OR UNTIL HEATED THROUGH AND CHEESE IS MELTED. SPRINKLE WITH TOMATO, LETTUCE AND REMAINING 1/4 CUP OF CHEESE.