Remove the stems and slice the bell peppers into narrow strips. Slice the onion. Add half of the peppers and onions to the bottom of the crockpot.
Arrange the chicken breasts on top of the veggies. Mix the chili powder, cumin, garlic powder, oregano, smoked paprika, salt, coriander, and pepper together in a small bowl, then sprinkle half of it over the chicken.
Add the remaining peppers and onions on top of the chicken, then sprinkle with the remaining spices.
Cover and cook on LOW for 4-6 hours or on HIGH for 2-3 hours.
Remove the chicken breasts and slice into thin strips. Drain most of the liquid from the crockpot, then add the chicken back with the peppers and onions. Toss together, then serve hot with your fajita toppings.