For stuffing, in a large skillet, cook mushrooms and onion in hot butter about 3 minutes or until tender. Stir in bread crumbs, thyme, 1/4 teaspoon salt, and a dash pepper. Set aside.
Trim fat from meat. On meaty side, cut a 3-1/2-inch-long and 1-inch-deep pocket above each rib, making 6 pockets. Spoon stuffing into pockets. Place meat, rib side down, in a shallow roasting pan. Sprinkle with salt and pepper. Insert meat thermometer into thickest part, not touching bone.
Roast in a 325 degree F oven for 1-1/4 to 1-3/4 hours or until thermometer registers 155 degree F. After 1 hour, cover loosely with foil to prevent overbrowning. Remove roast from oven. Cover with foil; let stand 15 minutes. (The meats temperature will rise 5 degree F during standing.) Slice roast between ribs. Serve with White Wine and Shallot Sauce. Makes 6 servings.
White Wine and Shallot Sauce
In a medium saucepan, stir together chicken broth; dry white wine, apple juice, or apple cider; and finely chopped shallot. Bring just to boiling; reduce heat. Simmer, uncovered, about 20 minutes or until reduced to 1 cup. Stir together softened butter and all-purpose flour; stir into wine mixture, 1 teaspoon at a time, stirring constantly. Continue cooking and stirring until thickened. Cook and stir for 1 minute more. Stir in snipped fresh thyme or sage or dried thyme or sage, crushed.