Start by slicing the bell peppers and onion. Mince the garlic and set all the vegetables aside in a mixing bowl.
In the same bowl, add the taco seasoning, ground cumin, salt, black pepper, and cayenne pepper (if using). Mix well to coat the vegetables with the spices.
Place the chicken breasts at the bottom of the slow cooker. Pour the undrained diced tomatoes over the chicken.
Add the seasoned vegetable mixture on top of the chicken and tomatoes, spreading it evenly.
Cover the slow cooker with its lid and set it to cook on low for about 6 hours.
After cooking, check to see that the chicken is tender. Using two forks, shred the chicken directly in the slow cooker and mix it with the vegetables.
Serve the chicken fajitas warm in tortillas and garnish with your preferred toppings such as sour cream, salsa, cheese, avocado, and cilantro.
For extra flavor, you can marinate the chicken in the spices for a few hours before cooking.
Serve with your favorite side dishes like Mexican rice or refried beans for a complete meal.
Leftover fajitas can be stored in an airtight container in the refrigerator for up to 3 days.