Place the chicken breasts in the bottom of the slow cooker in a single layer.
Scatter the sliced bell pepper and sliced onion evenly over the chicken.
Sprinkle the fajita seasoning across the peppers, onions, and chicken.
Pour 1 cup of salsa over everything.
Cover and cook on low for 6–8 hours or high for 3–4 hours.
Finishing Touches
Remove the chicken to a cutting board and shred it with two forks.
Return the shredded chicken to the slow cooker and stir to combine with the softened vegetables.
Warm tortillas, fill with the shredded chicken and veggies, and top with shredded cheese, sour cream, and avocado if using.
Notes
For a richer flavor, you can use chicken thighs. If you need a chunkier finish, swap salsa for diced tomatoes with green chiles. Store leftovers in an airtight container. Can freeze for up to 3 months.