Melt 2 tablespoons butter in a small frying pan over medium heat. (If a frying pan was used to cook the steaks, drain the fat from the pan and use it to cook the mushrooms.) Add the mushrooms, sprinkle lightly with salt, and cook them until they start to soften. As they start to release water, increase the heat. Add the cognac
and wines. Cook until most of the liquid has evaporated. Off the heat, stir in the mustard and the last tablespoon of butter. Season with salt and pepper.