Sauté chopped onion and garlic in 1 tablespoon hot oil in a large skillet 8 minutes. Stir in mint, orange juice, 1/2 teaspoon salt, and 1/2 teaspoon pepper; cook, stirring often, 5 minutes or until orange juice evaporates.
Butterfly roast by making a lengthwise and horizontal cut down center of roast, cutting to within 1/2" of other side. Open roast, and place between 2 sheets of heavy-duty plastic wrap; flatten to 1/2" thickness, using a meat mallet or rolling pin.
Spread onion filling over roast, leaving a 1" margin around edges. Roll up roast; tie at 1" intervals with string.
Combine remaining 2 teaspoons salt, 2 teaspoons pepper, rosemary, minced onion, and 1 tablespoon oil. Spread over roast. Place on a rack in a roasting pan.
Bake, uncovered, at 450° for 15 minutes. Reduce to 350°; bake 1 hour and 15 minutes or until a meat thermometer inserted into thickest portion registers 160°.
Remove string. Let stand 10 minutes before slicing. Garnish, if desired.