1. Heat grill to high and brush with oil. Mash rosemary, garlic and salt and pepper in a mortar and pestle, then stir in the oil. (Or finely chop the garlic, rosemary and salt with a large knife. Pu into a small bowl; stir in pepper and oil.) Spread evenly over both sides of chicken.
2. Grill 3-5 minutes per side until just cooked through. Transfer to a serving dish and drizzle with balsamic vinegar. Serve hot or at room temperature.
