Place bread cubes on a baking sheet; bake at 325° for 15 to 18 minutes or until lightly browned. Transfer bread to a large bowl, and set aside.
Remove casings from sausage; discard. Cook sausage in a large skillet, stirring until meat crumbles and is no longer pink. Remove sausage, reserving drippings in skillet.
Sauté 2 cups onion and next 4 ingredients in drippings over medium-high heat 8 minutes or until tender; stir in sausage, mustard, and next 5 ingredients. Add to bread cubes, stirring until blended. Set aside.
Place turkey breast halves flat on heavy-duty plastic wrap, skin side down. Remove tendons, and trim fat. From center of each breast, slice horizontally through thickest part of each side almost to outer edge; flip fillets over. Pound breasts to flatten and create even thickness. Sprinkle each breast evenly with 1/2 teaspoon salt and pepper.
Spoon half of stuffing mixture over each breast, spreading to within 1 inch of edges. Fold in sides of breast over filling; roll up over filling. (Rolls should be 10 to 12 inches long.)
Tie rolls securely in several places with heavy string. Place coarsely chopped onion, carrot, and celery in a large roasting pan; nestle quahogs into vegetables. Place turkey rolls, seam side down, over vegetables.
Bake at 425° for 30 minutes. Reduce heat to 350°, and bake 50 additional minutes or until thermometer registers 170°. Remove turkey from pan; set aside.
Remove and discard vegetables and quahogs, reserving drippings in pan. Place roasting pan on cooktop; add wine and chicken broth. Cook over medium heat, stirring constantly, 5 minutes or until liquid is reduced to 3/4 cup.
Combine cornstarch and 1 tablespoon water, stirring until smooth; whisk into broth mixture. Bring to a boil over medium heat, stirring constantly. Boil, stirring constantly, 1 minute. Stir in olives and rosemary. Spoon gravy over sliced turkey.
