Check the fish for bones and, if needed, remove with tweezers. To make the cure, put the lemongrass, 125g sugar, the salt, lime juice, zest of 1 lime, 1 tbsp rice vinegar, 1 garlic clove, half of the coriander and dill, including stems, into a food processor. Blitz until the lemongrass is finely chopped, then stir in a quarter of the sliced chillies.
Cross two large pieces of cling film in a roasting tin and spoon on a third of the cure, then top with the fish. Cover the fish with the rest of the cure. Wrap tightly and leave for 36-48 hrs, then scrape off the cure, rinse the fish in cold water and pat dry. The fillet will feel firm.
On the day of serving, start the other elements of the recipe. Shave the turnips into very thin slices using a mandoline, then plunge them into iced water with a dash of the vinegar. Leave for an hour, then drain well. Thinly slice the onion and mix with the turnip slices, 2 tbsp vinegar, 2 tsp sugar and a pinch of salt. Cover and steep for at least 1 hr, or chill for a few hrs.
Finely chop the remaining dill leaves and stir most of them into the crème fraîche with the remaining lime zest and a little seasoning, then chill.
For the dressing, stir together the remaining 2 tbsp sugar, 2 tbsp vinegar, another quarter of the sliced chillies and the crushed garlic and leave until the sugar dissolves.
Slice the fish into approx 20 slices, taking the knife down to the skin, then easing the blade underneath each piece to remove it.
When ready to serve, spoon and swoosh the dill crème fraîche down the middle of each plate, then top with overlapping salmon slices. Add 3 prawns per serving. Lift the salad from its juices and mound to one side. Scatter with a little extra dill and the remaining sliced chillies, then drizzle the dressing over the fish and prawns (or let your guests do that bit at the table).