1. Heat oven to 400°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray. Cut onion and bell pepper into even slices; place in baking dish.
2. Cut chicken breasts into thin strips; add to vegetables in dish. Sprinkle with seasoning mix; drizzle with oil. Stir until combined and pieces are coated.
3. Bake 35 to 40 minutes, stirring once halfway through baking, until chicken is no longer pink in center.
4. Spoon small amount of chicken and veggies onto each tortilla. Top each with sour cream, salsa and cilantro, or your favorite fajita toppings.