How to Make It
In a large bowl, whisk the yogurt with the green peppercorns, shallots, garlic, lemon juice, sugar and cumin. Stir in the cilantro and mint and season lightly with salt and pepper.
Transfer 1 cup of the yogurt sauce to a bowl; cover and refrigerate. Pour the remaining yogurt sauce into a large, resealable plastic bag. Add the lamb and turn to coat thoroughly. Seal the bag and refrigerate overnight.
Light a grill or preheat a grill pan. Remove the lamb from the marinade and thread the meat onto the skewers. Season with salt and pepper. Grill the lamb over high heat, turning, until charred in spots, about 7 minutes for medium meat. Transfer the skewers to a platter and serve with the reserved yogurt sauce.