Place the fish in a non-metallic dish and sprinkle with the lemon juice. Cover and leave in the refrigerator to marinate for 15–20 minutes.
Put the coriander, garlic and chilli in a food processor or blender and process until the mixture forms a paste. Add the sugar and yogurt and briefly process to blend.
Lay the fish on a sheet of foil. Coat the fish on both sides with the paste. Gather up the foil loosely and turn over at the top to seal. Return to the refrigerator for at least 1 hour.
Place the parcel on a baking tray and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 15 minutes until the fish is just cooked.