Brown ground beef, drain. Combine taco sauce and tomato puree. Line Dutch oven with aluminum foil. Place 2 tortilla shells in Dutch oven. Place 1/2 of ground beef on top, pour 1/2 taco sauce over top. Place 2 more tortilla shells on top and place in rest of beef and pour remaining taco sauce on top. Sprinkle with cheese. Cover and bake until cheese is melted. Variations: Add chopped onions, mushrooms or tomatoes to meat.