Peel and finely dice the shallots. Peel and wash the potatoes and cut into small cubes. In the meantime, let the puff pastry thaw at room temperature for about 10 minutes. Melt fat in a pot, fry shallots and potatoes in it, season with salt and pepper. Deglaze with stock and cream, bring to the boil, cover and cook for 18-20 minutes
Roll out the puff pastry on a floured work surface to form a rectangle (approx. 18 x 22 cm). Use flower cutters (2 to 4 cm Ø) to cut out about 20 flowers close together. Place the flowers on a baking tray lined with baking paper. Whisk the egg yolk and 1 tbsp. water. Brush the flowers with the whisk and bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 10 minutes. Remove from the oven and let cool on a cake rack
In the meantime, wash the lime thoroughly, grate dry and finely grate the peel. Mix with olive oil. Wash the chervil, shake dry and chop. Remove soup from the heat, add chervil and puree soup finely. Season again with salt, pepper and sugar
Add the crayfish meat, heat briefly. Serve the soup in deep plates sprinkled with lime oil. Garnish with puff pastry fleurons