Saut 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.
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 16