Combine garlic, rosemary, salt and pepper in small bowl. Mash with fork to form coarse paste. Place lamb in shallow baking dish. Spread half of garlic paste over each side. Cover and refrigerate at least 2 hours or overnight. Bring to room temperature before cooking.
Prepare barbecue (medium-high heat). Grill lamb until brown and crusty on outside and thermometer inserted into thickest part registers 130ºF for medium-rare, turning occasionally, about 25 minutes. Let stand 10 minutes. Cut on diagonal into thin slices and serve.