Peel and chop the potatoes. Fill a bowl with water, add lemon juice. Brush black salsifies thoroughly under running water, peel and slice diagonally. Immediately add to the lemon water
Onion peel, halve and finely dice. Heat butter in a pot. Fry diced onion in it. Add stock, bring to the boil. Drain black salsifies. Add potato and black salsify pieces to the stock and simmer for about 25 minutes. Remove some pieces of black salsify from the stock with a skimmer and set aside
Add crème fraîche and milk to the soup and puree everything. Season to taste with salt, pepper, nutmeg and lemon juice. Put the remaining black salsify slices back into the soup, bring to the boil
Meanwhile wash parsley, dab dry, pluck off leaves and chop finely and add to the soup. Cut salmon into strips. Arrange soup in bowls. Add salmon strips. Garnish with parsley leaves