Add the bell peppers and onions to the slow cooker. Toss them together with clean hands. Lay over the chicken breast tenderloins. Sprinkle over the salt, pepper, chili powder, garlic, and cilantro. Cut the lime in half and squeeze the juices over everything in the slow cooker. Drizzle over the oil next.
Place the lid on the slow cooker and cook on HIGH for 4 hours or LOW for 6-8. Do not open the lid during the cooking time.
Serve on warmed tortillas with desired toppings. Enjoy!