Method:
Soak bamboo skewers in water at least 15 minutes.
Meanwhile, put lamb, 1/2 cup of the yogurt, cilantro, onions, mint, and seasoning into a medium bowl. Combine gently but thoroughly, then divide into 8 balls. Shape each ball into a long sausage shape and slide skewers through the center, lengthwise. Flatten slightly and refrigerate for up to several hours until ready to grill.
Prepare grill. Brush lamb with oil and cook over hot coals about 10 minutes, turning to brown all sides. While skewers are grilling, mix together the remaining yogurt, tahini, lemon juice and garlic. Thin with water to make a smooth slightly thick sauce. Season with salt and pepper. Serve skewers with grilled pita or flat bread, drizzled with sauce.