Preheat oven to 425°. Line 2 sheet pans with foil and spray with cooking spray.
Toss bell pepper and onion slices with remaining olive oil and salt and pepper to taste (approximately 1 tsp salt and ½ tsp pepper).
Arrange chicken pieces in a single layer on baking sheet. Arrange bell peppers and onions on the second sheet. Roast about 15-20 minutes, until chicken is cooked through and veggies are soft. Remove chicken and turn oven to broil. Broil veggies for 2 minutes until they take on a little char.
Serve with warmed, toasted tortillas, sour cream, chopped cilantro, avocado or guacamole, and salsa, as desired.