Bring the soup and the Noilly Prat to the boil. Add the lentils and cook with the lid closed for about fifteen minutes until very soft.
In the meantime, cut the salmon fillet in half lengthwise. Cut the two halves diagonally into very thin slices. Leave the salmon to cool with the lid closed.
Cut the leek lengthwise into fine strips at the beginning, then diagonally into small cubes. Bring a little salted water to the boil, pour in the leeks and bubble for a minute. Pour off into a colander and quench when cool. Drain well.
Set aside 1 tablespoon of the cooked lentils per unit. Finely grind the remaining lentils together with the cooking liquid. Add the cream and the leek and let the soup boil repeatedly. Season with the juice of one lemon, salt and cayenne pepper.
Divide the salmon slices evenly into warmed soup plates. Cover with the boiling lentil soup and add the lentils set aside.