Effortless, really, and classic, these kebabs are best grilled over a mass of hot charcoal. If you are using bamboo skewers, soak them for half an hour first.
Grate the onions into a colander set over a bowl, sprinkle with salt, and leave to sweat for twenty minutes. Press the onions down with a spoon to extract all the juice, chuck the pulp and mix the juice with the garlic and the cumin seeds.
Stir in the lamb and marinade at room temperature for a few hours, then thread the meat onto skewers.