Wash the beef tongue and boil it in water until tender, roughly 2-3 hours
Remove the tongue from water, let cool slightly, then peel off the skin and cut into thin slices about ½ cm thick
Dice 2 onions and sauté in olive oil over medium heat until softened, roughly 5-8 minutes
Add 2 tablespoons tomato paste, 1 glass red wine, about 0.4 liters of the reserved cooking water, salt, pepper, and 1 teaspoon dried thyme; reduce over medium heat for 45 minutes
Stir in 2 tablespoons quince jelly and 2 tablespoons crème fraîche
Return the tongue slices to the sauce and heat through over medium heat for 2-3 minutes, then serve