Skin the fillet of salmon and remove the line of small bones that runs down the middle of the fillet using a pair of pliers. Cut off the dark meat on the skin side and discard. Cut the flesh into 5 mm/¼ inch cubes.
Cut the cucumber across its width into three equal pieces. Either cut these, having removed the seeds, into batons about 3 mm/⅛ inch wide, or turn them into barrel shapes, allowing five or six pieces to each portion.
Pick the dill of its stalks, saving four good sprigs to use as garnish. Break the rest up to make them a little finer. Wash the frizzy endive well and break up the leaves into large pieces.
Combine together the vinegar, oil and mustard to make a vinaigrette and season.
Butter a small saucepan, place in the cubes of salmon along with the vermouth and fish stock, and season. Cover with a tightly fitting lid and heat until the liquid begins to tremble, then reduce the heat and allow to cook for about 15 seconds. Drain the salmon from the stock, cover and keep warm. Reduce the stock over a high heat until it is syrupy. Add the cream and cucumber, bring back to the boil and reduce slightly until thickened.