Peel and finely chop the onion. Melt the fat in a pot. Fry the diced onion until transparent. Add flour and sauté. Deglaze with stock and ##milk## and simmer for about 5 minutes while stirring.
Meanwhile, wash the dill, dab dry, pluck the pennants and cut finely. Stir in horseradish and dill, season to taste with salt, pepper and possibly ##lemon##juice.