In large skillet, heat oil over medium-low heat. Add onion; sauté until soft, about 5 minutes; remove 1/2 cup of the onion; set aside. Add carrot to onion in skillet; cook and stir 1 minute. Add beef; cook and stir until browned, breaking up large chunks as you stir, about 10 minutes; spoon off any fat from pan. Stir in taco seasoning and 1/4 teaspoon salt; cook 1 minute longer. Add salsa. Cook and stir until hot and thickened, about 5 minutes.
In small bowl, mash together avocado, cilantro, lime juice, reserved remaining onion and 1/4 teaspoon salt. Divide lettuce equally among taco shells; top each with about 1/4 cup beef mixture. Divide 1-1/2 cups of the cheese into shells. Top each with avocado mixture; sprinkle with remaining 1/2 cup cheese.