Peel the onion. Peel and wash the potatoes. Dice both. Clean and wash the leek and cut the white and light green into fine rings.
Heat the oil in a large pot. Brown the onion, potatoes and leek for about 10 minutes. Season with salt, pepper and 1 pinch of sugar.
Deglaze with 1 l water, lemon juice, 200 g cream and milk. Bring to the boil, stir in broth and simmer covered for about 20 minutes. Puree the soup finely. Season to taste with salt and pepper.
Whip 100 g cream until semi-stiff. Wash the chives and cut into fine rolls. Serve the soup with 1 blob of cream and some chives. Serve hot or cold.