Trim any excess fat from the chicken breasts. Place the chicken in the bottom of a crock pot. Slice the mushrooms and the onions and place over the chicken. Sprinkle the onion soup mix over the chicken and vegetables. Add the chicken bouillon and some salt and pepper. Put the lid on the crock pot and let the chicken cook on low heat for 4-6 hours, or until fork tender. When the chicken is cooked, carefully remove the chicken breasts from the pot without shredding the chicken as best you can. Place the chicken onto a serving platter. Remove all of the juice and mushrooms from the pan and add 1/4 cup water, butter and 2 Tbs. cornstarch. Whisk together. Bring the mixture to a boil, add then add the half and half. Continue to whisk while bringing the mixture to a boil, until it starts to thicken. If the mixture gets too thick, add a little more half and half or water. Pour the mushroom sauce over the top of the chicken and garnish with fresh, chopped parsley. Serve over cooked noodles or mashed potatoes.