Place the fish in a non-metallic dish and sprinkle with the lemon juice. Cover and leave to marinate in the refrigerator 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. Chill for at least 1 hour.
Place the parcel on a baking sheet and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 15 minutes until the fish is just cooked through.
Garnish with coriander sprigs and sliced green chillies and serve immediately.