Using a pestle and mortar, grind the spices into powder. Put half of this mixture aside, mix the other half with the yogurt, oil, lemon zest and tomato paste.
Put the lamb steaks in a freezer bag, pour over the yogurt mixture, tie the bag and massage the meat in the marinade to coat them well. Chill in the fridge overnight or for a minimum of 2 hours before cooking.
Take the lamb steaks out of the bag and wipe off most of the marinade, then rub them with the spice mimxture you put aside.
Heat a grill, BBQ or griddle pan. Just before cooking, season the steaks with salt&pepper, then grill them for 3-5 minutes on each side, depending on the thickness of the meat and how well-done you like your lamb.
Rest for a couple of minutes before serving.