1. In each of 2 large skillets, melt 1½ tablespoons of the butter.
Add the onion to one skillet and the mushrooms to the other.
Season both with salt and pepper and cook over high heat until
the onion is softened and the mushrooms are browned and
tender, about 5 minutes. Off the heat, add the Cognac to the
mushrooms. Cook over high heat until evaporated, about 10
seconds. Add the mushrooms to the onion and wipe out the
skillet. Add the stock to the mushrooms and boil until nearly
evaporated, 5 minutes. Stir in the créme fraîche and mustard and
simmer until thickened, about 5 minutes; keep warm.
2. In the empty skillet, melt the remaining 1 tablespoon of butter
in the oil. Season the meat with salt and pepper and add it to the
hot skillet. Cook over high heat, turning once, until lightly browned
in spots but still rare, about 2 minutes. Scrape the meat and
any juices into the mushroom sauce and simmer just until heated
through, 1 minute. Serve right away, with the rice pilaf.
SERVE WITH Rice pilaf: Melt 2 tablespoons butter in a saucepan.
Add 1 small minced onion and a pinch of salt and cook over
moderately low heat, stirring, until softened, 8 minutes. Add 1 cup
basmati rice and cook, stirring, for 3 minutes. Add 2 tablespoons
dry white wine and boil until evaporated. Add 1½ cups water and
bring to a boil. Cover and cook over low heat until the water is
absorbed and the rice is tender, about 25 minutes. Fluff the rice,
cover and let stand for 5 minutes.