Wash the asparagus thoroughly and cut off the lower woody ends. Peel the white asparagus. Bring 2.5 litres of water, salt and sugar to the boil covered. Add asparagus skin and ends, bring to the boil, cover and simmer for 10-15 minutes.
Meanwhile peel the onion and dice very finely. Cut asparagus diagonally into thin slices. Heat the oil in portions in a large pan. Fry the asparagus in 3 portions. Pour the asparagus peelings into a sieve, collect the asparagus water and allow the peelings to drip off well.
(There should be just under 2 litres of asparagus water left).
Melt butter in a pot. Sauté onion in it until translucent, dust with flour and sauté. Gradually deglaze with asparagus water and cream and bring to the boil again and again. Season to taste with salt, stock, lemon juice and sugar.
Add asparagus to the soup, let it stand for a short time, garnish with salmon and sprinkle with pepper.