Cook ground beef and onion in a large skillet, stirring until beef crumbles and is no longer pink. Drain. Stir in tomatoes, seasoning mix, and 3/4 cup water; cook over medium heat 10 minutes or until liquid evaporates.
Stir in pinto beans and cheese spread; cook until cheese spread is melted. Serve over shredded lettuce in tortilla salad shells with desired toppings.