Cook the beef, onion and chili powder in a 10-inch skillet over medium-high heat until the beef is well browned, stirring often to separate meat.
Pour off any fat.
Stir 1/2 cup soup in the skillet and cook until the mixture is hot and bubbling.
Heat the remaining soup in a 1-quart saucepan over medium-high heat until hot and bubbling. Spoon the beef mixture into the taco shells. Top with the soup, lettuce and tomato.
