Wash lemon hot and cut in half. Wash and peel the asparagus and cut off the woody ends. Peel and halve the onion. Sweeten 1 litre of cold salted water with 2 tbsp. sugar. Add the asparagus peel, asparagus pieces, 1/2 lemon and onion and bring to the boil slowly. Simmer for about 15 minutes. Pour stock through a fine sieve and collect. Melt butter and crab paste. Stir in flour and sweat briefly. Remove the pot from the stove.
Cut the asparagus, except for the tips, into pieces of about 2 cm. Boil up the asparagus stock. Cook the asparagus pieces and tips for about 6 minutes. Pour the stock through a sieve and collect (approx. 750 ml). Put roux back on the stove. Add milk, broth and stock while stirring, bring to the boil and simmer for about 5 minutes while stirring. Heat the asparagus in the soup.
Wash the dill, shake dry, pluck flags from the stalks and cut finely, except for a little to garnish. Whip cream until semi-stiff. Squeeze juice from the remaining lemon half. Season soup with salt, sugar, pepper, lemon juice and serve. Sprinkle with crab meat and dill and garnish with dill.