Cook hamburger and chopped onion until meat is done; reserve as much grease as possible. Add taco sauce and water. Let simmer to blend flavors. In bottom of 9x13 pan, place tortilla chips keeping chips unbroken. Cover the chips with a layer of shredded lettuce 2 inches deep. Pour meat mixture over lettuce and top with cheese. Bake in a 350 oven for 15-20 minutes. Cut into squares and serve.
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