Wash the fish well, then pat dry with kitchen paper. Using a sharp knife, make deep, slightly diagonal slits across the fish on both sides at 2½cm (1in) intervals. Rub with ½ teaspoon salt, going well into the slits, stomach and cavities. Set aside for 10 minutes.

Combine the natural yoghurt, lemon juice, garam masala, cayenne pepper, ginger, garlic and ¼ teaspoon salt in a bowl and mix well. Rub most of this paste all over the fish, again going into all the slits on both sides and into the cavities. Retain about 2 tablespoons.

Place the fish on a rack set on a baking sheet (to catch the drippings) and set aside for 10 minutes. Meanwhile, preheat the grill, arranging a shelf so that the top of the fish will be a distance of about 13–15cm (5–6in) from the source of heat.

Drizzle half the oil over the top of the fish and grill for 9–10 minutes or until browned. Turn the sheet around halfway through this period to ensure even colouring. Now carefully turn over the fish. Spread the remaining spice paste on the second side (it will have lost some of its original marinade) and drizzle the remaining oil over the top. Grill this side for about 8 minutes or until it, too, is browned. Turn off the grill and heat the oven to 180°C/350°F/gas 4. Let the fish bake for 10 minutes. Serve immediately.