Very generously shower one (1 ¼ pound) boneless ribeye with salt on all sides. Let sit at room temperature as you prep other elements of your meal.

Preheat a cast iron skillet over high heat for 3 whole minutes.

Drizzle steak with a generous tablespoon of vegetable or olive oil. Add the steak to the skillet and sear, without moving, over high heat, until golden and crusty on the underside, about 3-4 minutes. Flip and continue to cook until an instant-read thermometer registers 105 degrees, about 2-3 minutes more. Pour off the hot oil into a heatproof bowl.

Immediately add the smashed garlic, the 3 tablespoons butter, and rosemary sprig. When the butter has melted, tip the skillet towards you. Using a large spoon, baste the top of the steak with the melted butter, continuously and quickly, for about 1-2 minutes, until the steak registers 112-115 degrees for medium-rare. Remove the steak to a plate to rest.

Reduce heat to medium-low and add ⅓ cup water to the skillet, scraping to incorporate the crusty pan drippings. Add 2 tablespoons Worcestershire and 1 tablespoon whole grain mustard. Add the remaining 3 tablespoons cold butter and continue to swirl pan until incorporated and emulsified, simmering until contents are reduced to a deliciously slick sauce. Pour any juices from the steak plate into the pan and season to taste with salt and pepper.

Slice steak and sprinkle with flaky salt. Divide between plates and serve with pan sauce.