Season the beef on both sides with salt and pepper. Set aside.
Next, add the oil to a large sauté pan over high heat and heat until it is smoking.
Add in the beef and sear for 2 minutes on 1 side and then flip over and sear for 1 more minute to achieve a rare to medium-rare internal temperature.
Remove the steaks and set them aside.
In the same hot pan, add the mushrooms and sauté for 6-8 minutes over high heat until well browned.
Push the mushrooms to one side of the pan, and on the other side of the pan sauté the shallots and garlic in the butter for 1 to 2 minutes or until lightly browned.
Mix everything and then remove the pan from the cooktop and add in the cognac.
Return the pan to the burner over high heat and flambe until all of the alcohol is cooked off.
Pour in the beef stock and cook over medium heat until the amount of liquid is reduced by one-half.
Finish by pouring in the cream, Worcestershire sauce, Dijon mustard, parsley, and lemon juice and stir until combined.
Place the beef back in the pan and cook over low to medium heat for 1 to 2 minutes until the beef is warmed.
Serve the sauce with the steak and garnish with sliced fresh chives.
Notes
Make-Ahead: This recipe is meant to be eaten as soon as it is finished cooking. If you have to keep it warm over very low heat for 15-20 minutes before serving.How to Reheat: Place the steak and sauce in a medium-size pan and cook over low to medium heat until warmed. You can also heat it in the microwave until hot.How to Store: Cover and store in the refrigerator for up to 5 days. It will freeze covered for up to 2 months. Thaw in the refrigerator for 1 day before reheating.There is no substitute for cognac other than brandy. The only thing you could use is white wine instead, which will not flambe. If you cannot have alcohol at all, simply skip this procedure.You can use wild mushrooms such as maitake, shiitake, or even oyster mushrooms in this recipe if you’d like.It is ok to sub out the beef stock with chicken stock. If you do not have access to either, then double the heavy cream amount and cook it until thickened before adding in the Worcestershire, Dijon, and lemon juice.You may need to sear the beef in batches if your pan is not big enough.If you like steak that is more well done, please sear longer or cook longer when reheating in the sauce.