Slice the bell peppers into strips and thinly slice the onion. Set aside.
Pat dry the chicken breasts and cut them into strips about 1 inch wide, ensuring they cook evenly and absorb flavors well.
In a small bowl, mix the fajita seasoning with a tablespoon of olive oil to create a fragrant spice paste.
Toss the chicken strips with the spice mixture until evenly coated, then set aside.
Place the sliced peppers and onions into the bottom of your slow cooker. Drizzle with the remaining olive oil and toss gently to coat.
Lay the seasoned chicken strips on top of the peppers and onions in the slow cooker.
Pour the lime juice over everything and add minced garlic if using, then cover the slow cooker with its lid.
Cook on low heat for 4 to 5 hours, until the peppers are soft and slightly charred, and the chicken is tender and easily shreddable.
Check the dish after 4 hours; the peppers should be caramelized and fragrant, and the chicken should shred easily with a fork. If needed, cook a little longer.
Use two forks to shred the chicken directly in the slow cooker or transfer to a serving dish. The peppers should be soft, slightly charred, and aromatic.
Serve the smoky chicken and peppers hot, with warm tortillas and your favorite toppings like cilantro or cheese for a complete fajita experience.