Trim excess fat from roast, and remove chine. Season roast with salt, thyme, and pepper. Place on rack in a shallow roasting pan. Insert meat thermometer, being careful not to touch bone or fat. Bake, uncovered, at 500° for 10 minutes; reduce temperature to 350°, and bake an additional 2 1/2 to 3 hours or until thermometer reaches 160°. Transfer roast to serving platter. Garnish with parsley and mushrooms. Reserve pan drippings for sauce. Slice and serve immediately with Madeira Sauce.