Prepare barbecue (high heat). Whisk hoisin sauce, soy sauce, Sherry, chili oil, rice vinegar, and finely chopped garlic to blend in shallow dish; mix in 1/3 cup thinly sliced green onions. Add lamb and turn to coat. Let marinate 20 minutes.
Brush grill rack with oil. Grill lamb to desired doneness, about 3 minutes per side for medium-rare. Place 2 lamb chops on each plate. Sprinkle lamb with remaining green onions and serve.