Mix together the cream cheese filling ingredients together in a bowl and set to the side.
Use a sharp knife to slice each chicken breast down the middle so that the chicken breast will open can be stuffed. Make sure you do not cut the breast all the way through. Repeat with all 4 breasts.
Stuff each breast with the cream cheese filling, spinach, and bell pepper slices. Season the chicken with salt and pepper then place the slow cooker in a single layer to cook. You do not need to add any additional liquid.
Cook the chicken on LOW for 2-3 hours of until the internal temperature is 70C/165F. Check the chicken after two hours, and then ever 20-30 minutes after that until the chicken is done. Serve and enjoy!