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Make the stuffing. Trim the spring onions and shred on the diagonal. Halve the chilli lengthways and scrape out the core and seeds. Cut this and the ginger into needle-thin strips. Crush the garlic.
Heat the oil in a wok, throw in the onions, chilli, ginger and garlic and stir fry over a medium to high heat for 3-4 minutes until softened. Tip in the creamed coconut and stir until it melts and thickens. Add the prawns and toss until they turn pink all over – about 2-3 minutes. Tip into a bowl, season, stir and cool.
Prepare the salmon. Pull out any pin bones from the flesh. Scrape the blunt edge of a large knife against the skin from tail to head to remove any scales. Rinse under cold running water and pat dry. With a very sharp knife or scalpel, score through the skin of one fillet in criss-cross diagonal lines.
Cut a sheet of parchment to fit the base of a roasting tray. Lay the paper on a work surface with one long side facing you. Cut four pieces of string, 30cm in length, and lay vertically at equal intervals on the paper. Put the unscored fillet, skin side down, across the string.
Spread the stuffing over the fillet, pressing down the prawns. Top with the scored fillet, skin side up. Bring the string round the fish and tie on top – position so you can cut either side to give portions of equal size. (The salmon can be made up to this stage the day before and kept, covered, in the fridge. Bring to room temperature before roasting.)
Put the empty roasting tray in the oven. Preheat the oven to fan 220C/ conventional 240C/ gas 9. Lift the salmon on the paper onto the tray. Roast for 15-20 minutes. Open one of the central slashes with a knife to see if the flesh is cooked. Heat the lime or lemon juice, honey and soy sauce in a small pan. Put the salmon on a serving dish and splash over a little sauce. Leave to rest for 5 minutes. Garnish with chilli and spring onion and serve with the remaining sauce.