Cut the lamb into mouthful sized chunks. Chop the mint and put half in a large bowl and half in a small bowl. Finely chop the chilli and add half to each bowl.
Stir 3 tbsp of olive oil into the large bowl, season, then add the lamb and turn until glistening and well coated (you can do this up to a day ahead).
Finely chop half an onion and add to the small bowl with the yogurt. Season, stir well, then chill until ready to eat (it will keep for a day in the fridge). Cut the rest of the onion into wedges and separate the layers.
Thread the lamb on to four large skewers, with onion in between. Preheat the grill or barbecue and cook the skewers for 6-8 minutes, turning until evenly browned. Serve the lamb with dollops of cooler, a leafy salad and hot new potatoes.