Clean the leek, cut in half lengthways, wash and cut into thin rings. Finely dice the onion. Melt the butter, fry the leek and onions until translucent, pour 1 liter of water over the vegetables, bring to a boil and simmer covered for about 20 minutes. Cut the cheese into flakes. Cut the ham into fine strips. Stir the grainy stock, white wine and sugar into the soup. Mix the flour with about 4 tablespoons of cold water and thicken the soup with it. Stir processed cheese into the soup and let it melt in it. Mix the ham strips and the cream into the soup and heat the soup once more, but do not boil. Arrange the soup and sprinkle each serving with grated parmesan.