Melt the butter and make a light roux with the flour. Deglaze with the milk while stirring with the whisk, slowly add the meat stock until the sauce has the desired consistency. Simmer on a low flame for about 10 minutes, stirring frequently.
Add the grated horseradish (quantity as desired), bring to the boil briefly, season with a little lemon juice, salt and sugar and serve hot with boiled beef.