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