Sauté onion in butter in a medium skillet until tender; add breadcrumbs, chopped mint, orange rind, egg, 1/2 teaspoon salt, and 1/4 teaspoon pepper, stirring well. Set aside.
Spoon breadcrumb mixture into the opening formed by boning. Fold roast over breadcrumb mixture; tie securely with string. Place roast in a roasting pan. Sprinkle roast with remaining salt and pepper. Insert meat thermometer, if desired.
Bake, uncovered, at 350° for 30 minutes. Combine wine and orange juice; pour over roast. Bake for 1 1/2 hours.
Score fat in a diamond design; coat with glaze. Bake until desired degree of doneness: 15 minutes or 140° (rare); 35 minutes or 160° (medium); 1 hour or 170° (well done).
Transfer roast to a warm serving platter; let stand 10 minutes before slicing.
Skim fat from pan drippings. Discard fat, and pour pan drippings over roast. Garnish with orange slices and mint leaves, if desired.