Place sliced bell peppers and onion in the bottom of the slow cooker. Drizzle with 1 tablespoon olive oil and toss lightly.
In a small bowl, combine fajita seasoning, remaining olive oil, minced garlic, and lime juice into a loose paste. Coat chicken breasts on all sides.
Place seasoned chicken on top of the vegetables and pour salsa over everything.
Cover and cook on low for 4 to 5 hours or high for 2 to 3 hours, until chicken reaches 165°F and shreds easily.
Remove chicken to a cutting board and shred with two forks. Return to the slow cooker and stir with peppers, onions, and cooking liquid.
Taste and adjust salt and lime juice. Broil filling 3 to 5 minutes on a foil-lined sheet pan if you want extra caramelization.
Warm tortillas and serve with desired toppings.
Notes
Layer peppers and onions under the chicken so the chicken does not sit directly in pooled liquid. Apply seasoning directly to the chicken for better flavor. Check early if your slow cooker runs hot so the chicken does not get stringy. Warm tortillas before serving so they fold without cracking.