Have the fishmonger cut the halibut steak into pieces about 5 x 4 x 2.5cm/2 x 1½ x 1 inch. Alternatively, you may do this yourself at home with a heavy cleaver that can hack through the bone. Leave the skin on.
Put the fish in a bowl. Sprinkle with ½ teaspoon of the cumin, ½ teaspoon of the coriander, ¼ teaspoon of the turmeric, ¼ teaspoon of the cayenne and ½