Remove all pieces of skin and tendons from the beef fillet, cut in half lengthways and cut across the grain. Melt 1 tablespoon of butter in the pan. Sweat the onions in it, cut the leek into strips, add and also, but only briefly, sweat. Now deglaze with wine. Add the cream, creme fraiche and peppercorns and bring to the boil. Season the sauce with salt, if necessary thicken slightly, bring to the boil again. Lightly salt the sliced meat and fry briefly and quickly on all sides in a second pan with a little hot fat. Add the sauce, toss and serve immediately.