Heat a 10-inch cast iron skillet over medium high heat until it is hot but not smoking, about 2 minutes. Generously season the steaks on both sides with salt and pepper. Add half the butter to the pan and set one steak directly on top. Repeat with the remaining butter and steak. Sear the steaks without touching for 2 minutes (or a minute or two longer if using bone-in), or until the steak releases easily from the pan and a golden brown crust has formed. Using tongs, hold the steak and sear both edges of the steaks for about 5 seconds. Turn the steak over and continue to sear until the steak reaches 115°F to 120°F for rare, 125°F for medium-rare, and 135° to 140°F for medium. Transfer the steaks to a warm plate to rest.
Remove the pan from the heat and carefully pour in the vermouth. Return to heat and scrape up any browned bits on the bottom of the pan with a wooden spoon. Cook until the wine is almost completely reduced, about 20 seconds. Stir in the mustard and heat for 15 seconds. Add the cream and any juices collected from the resting steak and stir to combine. Cook until the sauce coats the back of the spoon, about 10 seconds. Season with salt and pepper. Pour sauce over steaks before serving.
Notes
You can use boneless or bone-in steaks, increase cooking time for bone-in.From Martha Stewart's Cooking School