For the quenelles, put the salmon flesh (it must be totally free of For the quenelles, put the salmon flesh (it must be totally free of bone and skin) into a food processor with the egg white and half the cream and whisk until the mixture is very smooth. Pass through a sieve and put into the fridge for at least an hour. 10 minutes before you cook the quenelles, stir in the rest of the cream.
Blanch the cut potatoes in salted water for 3–5 minutes until just tender. Drain and refresh in cold water and put aside.
Keeping back about a quarter of the lettuce as decoration, cook the lettuce with the shallot in the melted butter very gently for about 5 minutes and then cover with the stock and the cream. Bring to the boil and then simmer for a half-hour. Put the broth through a sieve or food processor but don’t over-process – the soup is more visually appealing if still flecked with green.
Make the quenelles by poaching little teaspoonfuls of the salmon mixture in boiling salted water – they will take minutes to cook and will float to the surface when ready. Take them out with a slotted spoon and serve in the soup with the matchstick potatoes and decorated with the remaining thin strips of lettuce.