Sauté onion and celery in butter until softened, about 5 minutes. Stir into dry ingredients.
Add mushrooms, water chestnuts and poultry seasoning.
Slowly add the stock, stirring continuously. Make sure not to add too much stock. The mixture should be well moistened but not soggy. Add salt and pepper to taste.
Stuff turkey breast and put remaining stuffing in a baking dish and bake at 350° for 30 minutes. With poultry contamination a concern, make sure to remove all stuffing from turkey prior to storing remaining turkey in refrigerator.