Cook the sausage in a skillet, breaking it into small crumbles. Drain excess fat. Add onion and celery, and cook briefly. Let cool.
Cut sliced bread into small cubes, and put in a large bowl with chopped apple, chopped mushrooms, and poultry seasoning. Add sausage mixture to bread, and mix well. Using your clean hands is OK. Stir in chicken broth one cup at a time--you want it moist, but not too wet. Refrigerate until ready to use.
Divide the stuffing into 2 equal parts. Half will be enough to stuff a 6-pound turkey breast. With turkey breast-side-down, lightly mound stuffing between ribs. Cover with aluminum foil. Turn breast over, and place in roasting pan. Add stuffing to neck cavity, and cook the turkey breast as directed.
Freeze the remaining stuffing for another use, or bake in muffin cups for about an hour. Paper liners make it easy, and the muffin cups aid portion control.
Each serving contains about 128 calories, 8 g protein, 2.5 g fat, 13 mg cholesterol, 16 g carbohydrate, 3 g fiber, and 246 mg sodium.