Put the lamb into a deep bowl. Whizz the onion to a mush in a food processor with the oil, garlic, cumin, cayenne and a good grinding of black pepper.
Tip over the lamb pieces and toss around. Set aside for an hour if you can, but a minimum of fifteen minutes.
Heat the grill until very hot. Shake any excess marinade from the cubes then grill until slightly charred but still juicy inside- about 4-6 minutes- tossing them about from time to time to get all sides cooked.
Sprinkle with salt then stuff into a pitta bread and drizzle over yoghurt. Sprinkle the top with chopped raw onion and a dusting of cayenne.