Instructions

Heat oven to 375 degrees F. Have 2 baking sheets ready.
Put taco shells on 1 sheet; spread flour tortillas on the other. Sprinkle each tortilla with 1 tablespoon cheese.
Brown ground beef in a large nonstick skillet, breaking up chunks with a spoon. Stir in taco seasoning mix and 3/4 cup water. Simmer uncovered, stirring occasionally, 3 minutes, or until slightly thickened.
Meanwhile, put baking sheets in oven (it won't be at 375 degrees F yet; that's OK). Bake 3 minutes, or until cheese melts.
Center a taco shell in each flour tortilla and bring up sides of tortillas. Fill tacos with beef mixture, then shredded lettuce, the rest of the cheese, and the taco sauce and onion.