Remove tendons and skin from meat and freeze slightly in the freezer. This way, the meat can then be easily cut into narrow strips.
Fry the finely chopped onion in hot butter until light yellow, add the meat, dust with flour and fry briefly for 2 to 2 1/2 minutes, stirring continuously (the meat may still be pink inside!), season with salt and pepper, remove from the frying pan and keep warm. Deglaze roast stock with wine, fold in crème fraîche and briefly bubble up.
Pour the sauce over the warmed meat and bring to the table immediately.