Begin by slicing the boneless skinless chicken breasts into strips, ensuring they're evenly cut for consistent cooking. Next, slice the bell peppers and onion into thin strips to maximize their flavor diffusion.

In your slow cooker, layer the sliced chicken, bell peppers, and onions. Pour in the diced tomatoes or salsa, ensuring the mixture is well-distributed. Finally, sprinkle the fajita seasoning evenly over the top.

Cover your slow cooker and set it to low heat, cooking for 6 to 8 hours. The chicken is ready when it's tender and easily shredded with a fork.

Once the cooking time is up, remove the lid and give the mixture a good stir, then serve in warm tortillas, lettuce wraps, or over riced cauliflower.