Instructions:
- Return the skillet (used for searing steak) to medium heat. Add 1 Tbsp butter. If using mushrooms, add them now and sauté until browned and softened. Add the minced shallots and cook until translucent, about 2 minutes.
- Push the aromatics to one side. Add the remaining 1 Tbsp of butter. Once melted, whisk in the flour and cook for 1 minute (creating a blonde roux). Whisk in the Dijon mustard, ensuring it's fully incorporated.
- Pour in the brandy/cognac. Immediately stand back and carefully ignite the spirit with a long match or lighter held slightly away from the pan. Let the flames burn out naturally (this burns off the harsh alcohol). Ensure overhead extraction fans are OFF for safety.
- Once the flames subside, immediately stir in the Worcestershire sauce and lemon juice. Whisk in the beef stock gradually, scraping up any browned bits (fond) from the bottom of the pan. Bring to a gentle simmer.
- Reduce the heat to low. Stir in the heavy cream. Let the sauce simmer gently for 3–5 minutes, allowing it to thicken slightly enough to coat the back of a spoon. Do not allow it to boil rapidly after adding the cream.
- Taste and adjust seasoning with salt and plenty of freshly cracked black pepper. Stir in the fresh parsley. Pour immediately over rested steaks.