Stir together chicken and fajita seasoning in a large bowl until evenly coated.
Sauté onion in a nonstick skillet coated with cooking spray over medium heat 4 minutes. Add chicken mixture, and sauté over medium-high heat 4 minutes or until done. Add garlic; sauté 1 minute. Stir in 1/2 cup salsa. Cook over medium-high heat 2 minutes or until thoroughly heated.
Serve chicken mixture over tortilla chips and lettuce. Serve with desired toppings.