Peel the asparagus, cut off the woody ends. Boil up about 1 1/2 litres of water, season with salt, nutmeg and sugar. Cook the asparagus for 8-10 minutes. Peel, wash and roughly dice the potatoes. Remove the asparagus.
Cook potato cubes in boiling asparagus stock for about 5 minutes. Melt fat, stir in flour and sweat for about 5 minutes. Take out the potatoes. Measure 400 ml asparagus stock. Deglaze roux with wine, milk and asparagus stock. Simmer the béchamel for about 15 minutes, stirring occasionally. Cut off asparagus tips about 3 cm long. Cut remaining asparagus into short pieces. Bring the béchamel to the boil, stir in crab butter and season to taste.
Simmer the béchamel for about 15 minutes, stirring occasionally. Cut off asparagus tips about 3 cm long. Cut remaining asparagus into short pieces. Bring the béchamel to the boil, stir in crab butter and season to taste. Heat up the crabs, asparagus and potatoes for about 5 minutes. Arrange and garnish with chervil