Wash and peel the asparagus and cut off the woody ends. Put the asparagus skins and ends in a pot with 1 litre of cold water. Season with 2 tablespoons lemon juice, salt and sugar. Bring to the boil and simmer for about 15 minutes.
Peel and finely dice the shallots. Wash the dill, dab dry and chop finely, except for a little to garnish. Dice trout fillets finely. Mix remaining lemon juice, fish, shallots and about half of the dill.
Season to taste with pepper and chill.
Pour the asparagus shells into a sieve, collecting the stock. Pour the stock back into a pot and bring to the boil. Cut the asparagus spears into pieces and cook in the stock for 6-8 minutes. In the meantime melt fat, dust with flour and sweat briefly.
Pour asparagus into a sieve, collect the stock and measure 800 ml. Pour the stock into the roux while stirring, bring to the boil and simmer for 1-2 minutes. Season with salt, pepper and Pernod and refine with cream.
Add asparagus slices and heads to the soup. Place trout tartar in the middle of a plate and pour soup over it. Garnish with dill and sprinkle with lemon peel.