Wash and dry the produce. Thinly slice the onion and set it aside. Core, seed, and remove white ribs from both bell peppers, and then cut the peppers into thin slices. Set the onion and peppers aside.
Halve the chicken breasts lengthwise and place them in the crockpot. Cover the chicken in the fajita seasoning, and pour the salsa on top. Toss in the onion and peppers.
Cook on low for 7-8 hours or on high for 3-4 hours.
When the chicken is cooked, use two forks to shred the chicken. Gently stir the chicken to coat it in sauce. Place the chicken and vegetables on tortillas. Use a slotted spoon when serving to prevent the tortillas from getting soggy from excess liquid left in the crockpot. Enjoy!
Top fajitas with shredded cheese and sour cream.