Preheat oven to 450F/230C—only if you want medium-rare steaks.
To cook the filet steak/filet mignon, heat the oil a heavy cast iron skillet over high heat until the oil just begins to smoke—the oil will look shimmery like there’s waves running through it.
Place the steak in the pan and sear for 2-3 minutes without moving. Flip the steak over and add the butter and rosemary. Spoon the butter continuously over the steak cooking for 2-3 minutes. At this point, the steaks will be rare. If you like them that way, no further cooking is necessary. If you desire more doneness, transfer the steaks to a baking pan and place in the oven for 5-7 minutes for medium rare.
In either case, cover the steaks to keep warm and to rest while making the sauce.
To make the sauce, in a small saucepan, bring the stock to a boil and reduce by half.
Add the garlic, brandy, say sauce, mustard, cream and peppercorns. Season with salt to taste and cook to a consistency that resembles heavy or double cream. Stir in the parsley and serve over the steaks.