Preheat oven to 300°F. Place taco shells on baking sheet and bake until golden brown, about 5-10 minutes.
Season chicken with salt and pepper and spray large non-stick skillet with non-stick cooking spray.
Cook chicken over high heat stirring until browned on all sides (about 6 minutes).
Reduce heat to medium and add the garlic, taco seasoning and onion to chicken. Add 1/2 cup water and mix well making sure to really scrap the bottom of the pan (that’s where a lot of the flavor is!).
Cook chicken and onions for about another 4 minutes until onions are tender and sauce is thickening.
Stir in salsa, corn and beans (at this point sauce should be thickened, if not, cook for a few extra minutes).
Spoon about 1/4 cup mixture into each warmed shell, and garnish with tomatoes, lettuce, and 1 Tbs light ranch (or light sour cream) if desired.