Bring the butter to the boil in a saucepan until frothy. Add the flour, stirring constantly (shouldn`t have any more lumps). Don`t let it get dark! Immediately deglaze with the liquid from the salsify glass and keep stirring. Take the pot off the hotplate so that nothing burns. Add the cream or, alternatively, milk and season with vinegar (it is better to start carefully), salt, pepper, granulated stock, sugar and grated nutmeg. Put the pot back on the stove, add black salsify and let it get hot.
Wiener schnitzel and boiled potatoes go well with it.
The quantities given are very vague, as my grandma always cooked by feeling. But it also has to do with your own taste and can be adjusted accordingly.