Put 4 plates into the fridge together with the salmon fillet and leave them to get really cold. Shortly before serving, mix all the remaining ingredients together in a bowl.
Put the salmon fillet skin-side down on a board. Hold a long, thin-bladed knife at a 45o angle and, starting at the tail end of the fillet, cut the salmon into very thin slices. Lay the slices, slightly overlapping, on each chilled plate and spoon over the dressing.
Leave for 5 minutes before serving.