Peel and chop onion. Remove core and seeds from bell pepper and chop it. Put onion and green pepper in bottom of crockpot. Add green chilis.
Arrange chicken breasts on top of vegetables. Sprinkle fajita seasoning over the chicken, and top with one can of mild green enchilada sauce. Place lid over crock. Turn on high, and let cook for approximately 6 hours.
Remove chicken breasts from crockpot and shred them. Add chicken back to vegetables and mix all together.
Serve on warmed flour tortillas, with cheddar cheese, sour cream, and whatever else you like.