Pat steaks dry with paper towels and season generously with salt and pepper on both sides. Heat olive oil in a large skillet over medium-high heat until shimmering. Sear steaks for 3-5 minutes per side for medium-rare, or to your desired doneness. Remove steaks from the skillet and place them on a cutting board, tented loosely with foil, to rest. Keep any pan drippings in the skillet.

Reduce the heat to medium. Add 1 tablespoon of butter to the skillet (if needed, if there are not enough steak drippings). Add the minced shallot and cook for 1-2 minutes until softened. Add the minced garlic and sliced mushrooms (if using) and cook for another 2-3 minutes until fragrant and mushrooms are tender.

Carefully add the brandy or cognac to the skillet. If using a gas stove, remove the pan from the heat and tilt it slightly to ignite the brandy with the flame (be cautious, stand back). If using an electric stove, ignite with a long lighter. Allow the flames to die down completely (this burns off the alcohol, leaving the flavor).

Stir in the Worcestershire sauce and Dijon mustard. Pour in the beef broth and bring to a simmer, scraping up any browned bits from the bottom of the pan with a wooden spoon. Let the sauce reduce by about one-third, which should take approximately 3-4 minutes.

Stir in the heavy cream and the remaining 1 tablespoon of butter. Simmer gently for another 2-3 minutes, allowing the sauce to thicken slightly. Season with salt and freshly ground black pepper to taste. (Remember the steak drippings and Worcestershire are salty, so taste before adding more salt).

Remove the skillet from the heat and stir in the fresh chives and parsley.

Slice the rested steaks against the grain (if desired) or serve whole. Spoon the warm Steak Diane sauce generously over the steaks. Serve immediately.