How to Make It
In each of 2 large skillets, melt 1 1/2 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 crème fraîche and mustard and simmer until thickened, about 5 minutes; keep warm.
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.
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 1 1/2 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.