Cut each fillet in half, then rub all over with salt and pepper. Put the fish pieces in a large bowl and sprinkle over the lemon juice. Set aside to marinate for 1 hour.

Meanwhile, to make the batter, sift the flour, rice flour, turmeric and chilli powder into a bowl. Stir in the water until the mixture forms a smooth batter.

Remove the fish from the marinade and pat dry with kitchen towels. Dip each fish piece in the batter and set aside.

Fill a deep-frying pan one-third full with vegetable oil and heat until it reaches 180°C (350°F) on a deep-fat thermometer, or until a small cube of stale bread dropped into the oil turns golden in 55 seconds. Carefully lower the fish pieces into the oil, a few at a time, and fry for 5 minutes, or until they are crisp and golden brown. Remove from the oil and drain on kitchen towels.

Transfer to a warmed serving dish and serve at once.