Season the steak well with salt and pepper. Heat a cast-iron skillet over high heat and add the oil. Sear the steak until nicely browned, about 2 to 3 minutes. Flip over and add garlic, thyme, and butter. Using a spoon, baste the steak with melted butter, once the steak is cooked to the desired temperature, remove from heat and let rest. To the same pan, add mushrooms and shallot and cook for a few minutes. Deglaze with brandy then add Worcestershire sauce and mustard. Add heavy cream and let simmer to thicken. Remove from heat and season with salt and pepper. Place the steak on a plate and serve with mushroom sauce.