Heat butter in a pot. Dust with flour and sauté until light yellow. Gradually stir in 1 l water. Stir in the stock, bring everything to the boil and simmer at low heat for about 5 minutes.
Meanwhile grate the cheese. Stir cheese and cream into the soup. Bring the soup to the boil again and season to taste with salt and pepper. Wash the lovage, chop finely and sprinkle over the soup.