Put the fresh coriander, mint, garlic, ginger, chillies, cumin, ground coriander, sugar, lime juice and coconut milk into a food processor or blender and blend until fairly smooth. Season and set aside.
Place each salmon fillet on a square of banana leaf and spoon some of the herb and spice mixture over it. Carefully wrap the fish in the leaf to make a neat parcel and secure with wooden skewers. If the banana leaves are difficult to handle, dip them in boiling water for 15–20 seconds and they will become more supple.
Place the parcels on a large baking sheet and bake in a preheated oven at 200°C (400°F), Gas Mark 6 for 15 minutes.
Remove the parcels from the oven, place on a serving plate and open the packages at the table.