Finely dice the onion and fry in hot fat, dust with flour and brown the roux well. Deglaze with the meat stock while stirring with a whisk, bring to the boil and season with salt and pepper.
Peel the raw potatoes and cut into thin slices. Add to the sauce and cook until tender for about 20 minutes. Season to taste with salt, pepper, vinegar and nutmeg and let it steep for a short time.