2)Heat 2 tablespoons of oil in a medium saucepan on the grates of the grill. Add the onion and cook until soft. Add the chilli powder, garlic, ginger and mustard seeds and cook for 1 minute. Add the vinegar and sugar and cook until the sugar dissolves and the mixture thickens slightly. Stir in the nectarines and cook for 1 minute. Season with salt and pepper. Transfer the mixture to a bowl to cool. Stir in the raisins, coconut and mint. Let cool to room temperature before serving.
3) For the Skewers: Heat the grill to barbecue to high. Combine the coriander, cumin, mustard seeds and black peppercorns in a small skillet on the grates of the grill and cook, shaking the pan, until lightly toasted, about 1 1/2 minutes. Transfer to a coffee grinder and process until finely ground. Transfer the powder to a bowl and add the chilli, turmeric, garlic, ginger, the oil and vinegar. Stir to form a paste. If the mixture is too dry, add a few tablespoons of water to loosen.
4) Place lamb in a large bowl, add the curry paste and toss to coat each piece of lamb. You can grill immediately or cover and refrigerate for up to 4 hours to allow the flavors to intensify.
5) Divide the meat between the skewers and season with salt. Grill, turning occasionally, until the meat is golden brown, slightly charred on both sides and cooked to medium-rare, about 2 to 3 minutes per side. Drizzle with olive oil and serve with pita bread and chutney.