Begin by slicing the boneless, skinless chicken breasts into thin strips, which will help them cook evenly and absorb flavors. Next, wash and slice a vibrant mix of bell peppers and onions, creating a colorful medley.

Place the sliced chicken, bell peppers, and onions in the slow cooker. Pour in the diced tomatoes or salsa over the top, making sure the ingredients are evenly distributed. Sprinkle the fajita seasoning generously across the mixture. Gently stir everything together until well combined.

Cover the slow cooker with its lid and set it to low heat for 3 to 4 hours, or high heat for 2 to 3 hours. The chicken should be thoroughly cooked and tender.

Once the cooking time is complete, carefully uncover the slow cooker, and avoid keeping it on the warm setting. Using a sturdy spoon, gently stir the mixture one last time. Serve the fajitas hot with warm tortillas or crisp lettuce wraps.