Add the beef stock to a saucepan and reduce over medium-high heat to 1/2 cup, about 1 hour.
Sprinkle the steaks on both sides with the salt and pepper. In a large skillet or cast-iron pan over medium-high heat, heat 2 tablespoons of the butter and the olive oil. When the butter has melted and the oil shimmers, add the steaks. Brown the steaks on both sides, 3 minutes per side. Transfer the steaks to a plate and set aside, lightly covered with foil.
Add the mushrooms and shallots to the pan and cook for 2 minutes, stirring frequently, adding oil if needed. Add the garlic. When the garlic is lightly colored, add the brandy (be careful, it can ignite). Add the wine, mustard, Worcestershire and the reduced beef stock. Simmer for 2 to 3 minutes more. Return the steaks to the pan and finish cooking them to the desired temperature, 2 to 4 minutes, depending upon the size of the filets and desired temperature/doneness. For medium-rare, cook to 135 degrees F on an instant-read thermometer. Add the remaining 2 tablespoons butter to the sauce to melt.
To serve, place a steak on each plate and pour the sauce over the steak.