Cut meat. Melt 1 tablespoon of butter in the pan. Sweat the onion strips in it, add the leek strips and also sweat (only briefly!). Deglaze with white wine. Add the cream, crème fraîche and pepper and bring to the boil in the pan. Season the sauce with salt, if necessary thicken slightly with cornstarch, bring to the boil again.
Season the sliced meat lightly with salt and fry briefly and quickly on all sides in a second pan with a little hot oil. Add the sauce, toss and serve.