Heat oven to 250°F. In medium skillet, brown ground beef and onion over medium heat for 8 to 10 minutes or until beef is thoroughly cooked, stirring frequently. Drain.
Stir in chili powder, salt, garlic powder and tomato sauce. Reduce heat to low; cover and simmer 10 minutes.
Meanwhile, place taco shells on ungreased cookie sheet. Heat at 250°F. for 5 minutes.
To assemble tacos, layer beef mixture, cheese, lettuce and tomatoes in each taco shell. Serve with salsa; top with sour cream.