Preheat oven to 400F. Spray a 9x13 baking dish with nonstick spray.
Heat olive oil over medium heat in a medium skillet.
Add onion to skillet and cook for 2-3 minutes, or until the onion is translucent and fragrant.
Stir in the chicken, taco seasoning, tomatoes (FULLY DRAINED), and green chiles (FULLY DRAINED) *see note. Stir to combine fully. Reduce to simmer and allow to cook for 5-8 minutes.
Place the taco shells in the baking dish, standing up. I was able to fit 10 taco shells in the dish by adding 2 on each side.
Bake the taco shells for 5 minutes by themselves to allow them to crisp up. Remove from the oven.
Spoon 1 tablespoon of beans into the bottom of each taco shell. Top with the chicken mixture, almost to the top of each shell.
Sprinkle each shell generously with shredded cheese, the more the better!
Bake for 7-10 minutes or until cheese is fully melted and the edges of the shells are browned.
Remove from the oven and top with your favorite items such as jalapenos, sour cream, cilantro, and salsa.