Season the chicken with salt and pepper on both sides.
In a skillet heat the butter with oil. Brown the chicken quickly on both sides, (if you are starting your crockpot in the morning, chop onion and brown your chicken the night before, and leave in the fridge until the morning, warm in the microwave for 5 minutes before adding to the crockpot to start the cooking process).
Place the chicken pieces in the crockpot, then sprinkle the bell pepper (if using) over the chicken.
In a large bowl, stir together all the remaining ingredients (except the heavy cream); pour into the crockpot over the chicken pieces.
Cover, and cook on LOW for 8-10 hours, or on HIGH for 4-5 hours.
Just before serving, stir in the heavy cream (if using).