1. Heat four tablespoons of the butter in a skillet, add the onion and cook two minutes. Add the mushrooms and celery, cover and cook five minutes. Remove the cover and continue cooking until most of the moisture has evaporated. Add the parsley and salt.
2. Preheat oven to moderate (350° F.).
3. Place the chops on a board. With a sharp knife, cutting from the bone toward the center, make a pocket in each chop and stuff with two tablespoons of the mushroom mixture. Fasten securely with toothpicks.
4. Dust the chops lightly with flour and sprinkle with pepper.
5. Heat two tablespoons of the butter in the skillet, add the chops and brown on both sides. Remove to a casserole, add the sherry, cover with waxed paper and a lid and bake until tender, about thirty minutes.
6. While the chops are baking, make the sauce in the pan in which the chops were browned. Melt the remaining butter, add one tablespoon flour and blend. Add the stock and bring to a boil, scraping loose all the brown particles. Cook, stirring, until thickened. Stir in sour cream. Remove from the heat.