Soften the tongue in salted water, skin and cut into slices.
Melt the butter in a saucepan, sauté the onion and flour in it and extinguish with boiled down tongue stock (about a quarter of a liter). Add lemon slices to the sauce and season with nutmeg and white pepper. Add white wine to taste. Place the slices of tongue in the sauce and simmer gently for a quarter of an hour. Refine with whipped cream before serving.