Slice onions and peppers. (I used half of each color of pepper, but you are welcome to just use all green, all red or a combination of the colors available to you.)
Add onions and peppers to a large bowl. Toss with olive oil and ⅔ of the fajita seasoning. Set aside.
Spray a cold crockpot with nonstick cooking spray and add the chicken. Sprinkle with remaining fajita seasoning.
Top chicken with onions and peppers. Cover and cook on high for 2-3 hours or until chicken is thoroughly cooked and peppers have softened (but not mushy).
Shred chicken and serve with tortillas and any other desired fajita toppings: tomatoes, salsa, avocado, guacamole, rice, beans, cilantro.