Place chicken breasts in a slow cooker. Add fajita skillet sauce, taco seasoning, and chilis. Cook on LOW for 6 hours. Shred chicken with a fork.

Place taco shells in a casserole dish. Add shredded chicken to the taco shell. Add some red onions and a generous sprinkling of cheese. Bake in a preheated 375°F oven for 15 minutes or until the cheese is melted. Top with lettuce, salsa, sour cream, guacamole, and cilantro.