Defrost the puff pastry. Peel onion and potatoes. Wash the potatoes. Dice both. Heat butter in a pot. Sauté onion and potatoes. Pour water and cream on well 3⁄4 l
Bring to the boil, stir in broth and simmer covered for about 20 minutes.
For the Fleurons puff pastry, roll out to a rectangle (approx. 18 x 22 cm) on some flour. Cut out the flowers close together with a flower cutter. Place on a baking tray lined with baking paper.
Spread with whisked egg yolk. Bake in a hot oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 10 minutes. Let it cool down.
Mix lime zest and oil. Wash and chop the chervil. Remove soup from the stove. Add chervil and puree everything with a chopping stick. Heat the soup again. Season to taste with salt, pepper and sugar.
Heat the crab meat in it. Serve the soup with lime oil and fleurons.