Slice the boneless, skinless chicken breasts into strips for even cooking.

Chop your bell peppers and onion into bite-sized pieces.

In your slow cooker, place the sliced chicken followed by the chopped bell peppers and onions.

Pour in the diced tomatoes and sprinkle your fajita seasoning over the top.

Cover the slow cooker and cook on low for about 4 hours, or on high for 2-3 hours, until chicken is cooked through.

If using the Instant Pot, add the same ingredients plus chicken broth and cook on high pressure for 10 minutes, followed by natural release for 5 minutes.

Once cooked, shred the chicken in the crockpot if desired and serve in lettuce wraps or warm tortillas.