Prepare the Steaks: Season the filet mignon steaks generously with salt and pepper on both sides.
Sear the Steaks: In a large skillet, heat the olive oil over medium-high heat. Once hot, add the steaks and sear for about 3-4 minutes on each side for medium-rare, or to your desired doneness. Remove the steaks from the skillet and set aside to rest, covering them with foil to keep warm.
Sauté Aromatics: In the same skillet, add the butter. Once melted, add the minced shallot and garlic, sautéing for about 1-2 minutes until fragrant. If using mushrooms, add them now and cook until softened.
Deglaze the Pan: Carefully pour in the brandy or cognac, scraping the bottom of the skillet to release any browned bits. Let it simmer for a minute to cook off the alcohol.
Make the Sauce: Stir in the Dijon mustard and heavy cream, mixing well. Allow the sauce to simmer for about 3-4 minutes until slightly thickened. Season with salt and pepper to taste.
Serve: Slice the steaks and arrange them on plates. Drizzle the sauce over the top and garnish with freshly chopped parsley. Serve immediately with your choice of sides. Enjoy!