After cooking your steak, remove from the pan and leave to rest whilst making the sauce.
In the pan add in your butter and shallots. Place on medium high heat and sauté for roughly 2 minutes or until translucent. Add in the garlic and button mushrooms and sauté for a further 2 minutes.
Once the mushrooms are cooked and lightly caramelised, deglaze the pan with the cognac. If using a gas stove, slightly tilt your pan to allow the alcohol to alight from the flame and flambe. If you are not using gas, allow the alcohol to cook off.
Once the cognac has reduced by more than half add in the beef stock, Worcestershire sauce, and mustard. Cook for 3 minutes and allow the sauce to slightly thicken.
Stir in the cream and bring to the simmer. Turn off the heat, fold through the parsley and season with salt and pepper to taste. A little lemon juice or vinegar can be nice in there too. If your resting steak has let out any juices, stir it through your sauce.
