Season the meat with salt and pepper and fry all over in hot clarified butter. Pour in the broth and stew for 40 minutes.
In the meantime, wash the leek and cut into rings. After 40 minutes, stir in the white wine, mustard and crème fraiche, then add the leek. Cover everything and cook for another 20-25 minutes. Then remove the meat and cut it into slices. Season the sauce with salt and pepper, then sprinkle in the sauce thickener and bring to the boil again.