Mix chicken, 2 tablespoons olive oil, garlic, mint, oregano, salt and pepper in a medium bowl. Let marinade for 30 minutes, Whisk remaining 2 tablespoons oil and lemon juice in a small bowl
Prepare barbecue ( medium high heat ) Pull off large mint leaves from stems. Alternate chicken, onion, and mint leaves on skewers. Sprinkle with salt and pepper. Grill until chicken is about cooked through, turning and basting occasionally with oil-lemon mixture about 9 minutes. Serve with Greek salad.