Brush roast with oil; sprinkle on all sides with 1 teaspoon salt and 1/4 teaspoon pepper. Place roast, bone ends up, in a shallow roasting pan. Insert meat thermometer, making sure it does not touch fat or bone. Bake at 475° for 15 minutes; reduce oven temperature to 325°, and bake 1 hour and 15 minutes.
Meanwhile, brown sausage in a large nonstick skillet, stirring until it crumbles. Remove from skillet, reserving 1 tablespoon drippings in skillet; drain. Cook onion, celery, and garlic in skillet over medium-high heat, stirring constantly, until tender; remove from heat.
Combine sausage, onion mixture, bread cubes, and next 6 ingredients, mixing well. Pour half-and-half over stuffing, stirring gently until blended. Spoon 3 cups stuffing into center of roast, mounding slightly. Cover stuffing and exposed ends of ribs with aluminum foil; spoon remaining stuffing into a greased 11 x 7 x 1 1/2 baking dish.
Bake roast and dish of stuffing at 325° for 40 minutes or until thermometer registers 160°. Transfer roast to a large serving platter; remove foil. Let stand 10 minutes before carving. Garnish, if desired.
*To roast your own chestnuts, you'll need 1 pound fresh chestnuts. Cut a slit in each chestnut shell. Place on an ungreased baking sheet. Bake at 400° for 15 minutes; cool. Discard shells.