Wash the potatoes and cook covered for about 20 minutes. Quench and let cool down a little. In the meantime peel and finely dice the onions. Wash chives, shake dry and cut into small rolls.
Heat butter in a large pot. Fry the onions in it until transparent. Dust with flour and sauté until light brown. Add 3⁄4 l water, milk and cream while stirring constantly. Add stock. Bring to the boil while stirring and simmer for about 5 minutes. Season with salt, pepper and 1 pinch of sugar. Keep warm.
Peel the potatoes, cut them into smaller pieces if necessary and heat them up in the sauce. Fold in chives.