Brush the nectarines with oil and season with salt and pepper. Grill, cut side down until lightly golden brown, about 3 minutes. Turn the nectarines over and grill until just cooked through, about 2 minutes longer. Remove from the grill and coarsely chop.
Heat 2 tablespoons of oil in a medium saucepan on the grates of the grill. Add the onion and cook until soft. Add the chile, 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 to a bowl, cool, and stir in the raisins, coconut and mint. Let cool to room temperature before serving.
For the skewers:
Heat the grill to high. Combine the coriander, cumin, mustard seeds and black peppercorns in a small skillet on the grates of the grill and grill until lightly toasted, about 1 1/2 minutes, stirring a few times; transfer to a coffee grinder and process until finely ground and place in a bowl. Add the ancho, turmeric, garlic, ginger, 1/4 cup canola oil and vinegar and whisk until it forms a paste. If the mixture is too dry, add a few tablespoons of water to loosen it to a paste.
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.
Skewer 2 pieces of lamb onto 2 skewers so that the meat lays flat on the grill. Season both sides of the lamb with salt and grill until golden brown and slightly charred on both sides and cooked to medium-rare doneness, 2 to 3 minutes per side. Drizzle with olive oil and serve with pita bread and chutney.