Begin by stewing the chicken and saving the broth. Take the chicken off its bones and place it in a greased 13 x 9 baking dish. Cook the spaghetti in the chicken broth. While the spaghetti is cooking, sauté the green pepper, onion and celery in butter until it is tender. Add cream of mushroom soup to the green pepper, onion and celery mixture and simmer until the spaghetti is done. Drain the spaghetti and pour it over the chicken, then pour the soup mixture over the spaghetti. Mix all of the ingredients together and then cover with grated cheese. Place aluminum foil over the baking dish and bake at 350 degrees for 45 minutes to 1 hour or until the cheese is melted and the casserole is piping hot. Note: This casserole can be prepared and frozen in the dish, but the cheese should be left off until it is ready to bake.