Take four squares of foil large enough to make a bag to hold the individual pieces of salmon and butter each square using all the butter. Place each piece of salmon on one of the buttered halves, season, and divide the vegetables out equally over the portions of salmon. Place a sprig of thyme on each. Fold the other half of the foil over and tightly seal by folding over the edges two-thirds of the way round. Pour in the fish stock and the vermouth and then seal completely.
Place the four bags on a baking sheet and cook in the