Dice the onion and sauté in the butter. Sprinkle with a little powdered sugar and mix well. Deglaze with cognac and simmer briefly.
Pour in the beef stock and add the pickled pepper. Reduce to half. Add the cream and bring to the boil again. Pass through a sieve. Season the sauce with salt, pepper and, if necessary, a dash of cognac. Heat again and stir in the crème fraîche. Thicken with a little starch.