Method:
Put vinegar, maple syrup, 1/4 cup of the oil, salt and pepper into a wide, shallow dish and whisk to combine. Add chicken, turn to coat, cover and refrigerate for at least 2 hours or overnight, turning several times.
Prepare a charcoal grill or heat a gas grill to medium high heat. Toss peaches and onions with orange juice, remaining 2 tablespoons oil, chopped rosemary, salt and pepper in a large bowl; set aside. Remove chicken from marinade and grill about 5 minutes per side, or until just cooked through. Transfer to a large plate and cover to keep warm. Grill peaches and onions until tender and slightly charred, then transfer to plate with chicken. Garnish with rosemary sprigs and serve.