Preheat oven to 350° F. Cook steaks as directed on package, cooking onion along with the steaks in a large skillet. Stir in tomatoes, beans, taco seasoning and corn. Simmer 5 minutes.
Arrange taco shells in shallow baking dish. Spoon steak mixture into shells. Top with shredded cheese. Bake for 10 minutes. Top with shredded lettuce, sour cream and salsa if desired.