1. Spray the grill rack with nonstick spray; preheat the grill to high or prepare a hot fire.
2. Toss the lamb, onions, garlic, oil, lemon juice, and salt in a large bowl. Thread pieces of lamb and pearl onions alternately on 8 metal skewers. Place on the grill rack and cook, turning frequently, until the lamb is browned and cooked through, for 4-5 minutes.
3. Sprinkle the lemon zest and mint over the kebabs just before serving.