For the chervil soup, first prepare the pastry to serve with it:
To do this, defrost the puff pastry. Spread the pastry on a little flour and cover with slices of smoked salmon. Roll up from the narrow side, cut into slices about 1 cm wide with a sharp kitchen knife. Cover a baking tray with baking paper and place the snails on it. Bake in a preheated oven at 220° C for about 15 minutes until lightly browned.
For the soup, heat the butter in a large saucepan and sauté the finely chopped onion until translucent. Peel the potatoes and chop them into pieces, sauté briefly, then pour in the soup and bring to a boil.
Wash the chervil, pluck off the leaves and put a few leaves aside for decoration. Finely puree the rest with cream in a tall container.
When the potato is soft, puree the soup, add the chervil top mixture and bring to a boil one more time, blend again.
Decorate the chervil soup with chervil leaves and serve with the salmon snails.