Heat a cast iron pan over high heat. (let it start to smoke)
Season steaks with salt and pepper. Add oil or butter into the pan then add the steaks. Cook each side for 4-5 minutes (for a medium-rare). Remove from the pan and tent with foil. Reduce heat to a medium temperature.
Whisk broth, water and Dijon mustard in the pan. Scrape up any brown bits in the pan with a wooden spoon. Add in flour and garlic while continuing to whisk. Add Marsala to the pan and bring to a boil then quickly reduce temperature and let thicken for 1 minute while stirring frequently.
Plate the filet and drizzle Marsala sauce over then enjoy with your favorite sides!