Boil the potatoes in their skins until just before they reach the boil, peel them and cut them into slices. Chop the shallot very finely and sauté in the butter until translucent, add the flour and sweat for about five minutes, stirring constantly. Deglaze with the stock and cream and reduce.
Add the mustard, some nutmeg and pepper. Now add the potato slices and let them steep for about 20 minutes. Stir again and again so that nothing sticks. Now fold in the sour cream and simmer for another 5 minutes.
We used to have fried blood and liver sausage with it. I prefer meatballs myself.
Incidentally, it tastes much better when it is warmed up.