In a large skillet, brown the lean ground beef and the chopped onion. Drain off the fat.
Stir in the tomato sauce and taco seasoning. Bring the mixture to boiling. Reduce the heat and simmer, uncovered, for 5 minutes. Remove the skillet from the heat.
Stir together the condensed soup and milk. Spoon half the soup mixture into a 13x9x2-inch baking dish.
Cut the tortillas in half. Use 12 halves to place over soup in dish.
Spoon meat mixture over tortillas. Top with remaining tortillas.
Spoon remaining soup mixture over the tortillas. Top with the cheese.
Bake the casserole in a 350 degree F oven for 30 minutes or until heated through. Makes 6 servings.