Cut the lamb into cubes (1½-2 cm / ½ in). Put into a bowl. Mix with onion and salt. Leave to marinate for at least 40 mins.
Light up charcoal in your grill. While waiting for the flame to subside, thread the lamb onto the skewers. Evenly distribute the lean pieces and fatty ones (discard the onion).
When the charcoal glows and there’s no more visible flame, place the skewers over the cooking grid. Flip frequently to sear the meat evenly.
Once the meat turns pale, sprinkle chili powder and cumin powder all over.
Divide the skewers into two parts and hold them with each hand. Dab and rub one lot over another. This will help the oil generated from the fat to flow onto the lean meat.
Continue cooking until the meat browns evenly and there’s no pinkness on the surface (do not overcook). Remove from the heat and serve immediately.