First, season the steaks with salt and pepper.
Next, add the steaks and butter to the hot skillet, and cook 4-5 minutes per side.
Next, sauté the onion and garlic in butter until softened. Then add the mushrooms.
Next, add the Cognac and cook 1-2 minutes until evaporated.
Next, add the beef broth, Worcestershire sauce, and Dijon mustard. Simmer for 3-5 minutes.
Next, stir in the heavy cream and simmer for 2-3 minutes.
Next, add the steaks to the sauce, and cook through for 2 minutes to warm.
Serve the steaks and sauce immediately, with buttermilk mashed potatoes.