Drain the salsify and collect the water for the sauce.
Heat the margarine in a saucepan and add the flour. Stir quickly. Now add the salsify water and keep stirring. Then stir in the milk and cream (or sour cream). Likewise the egg yolk. Stir in a little more milk depending on the desired consistency.
Finally season well with salt, pepper, (about 1 teaspoon) sugar and plenty of nutmeg. Put the salsify back into the sauce.
We usually have this dish with potatoes and meatballs.