How to Make It
Preheat the oven to 450°. In a bowl, stir the butter, anchovies, garlic and 2 tablespoons of the parsley. Season with salt and pepper and mix well.
In a large cast-iron skillet, heat the oil until shimmering. Season the steak with salt and pepper. Add the steak and rosemary to the skillet and sear over moderately high heat until the steak is browned, 2 minutes per side. Add the radishes and half of the butter and roast in the oven for 18 to 20 minutes, basting every 3 minutes, until an instant-read thermometer inserted in the meat registers 125° for medium-rare; transfer to a board to rest for 15 minutes. Discard the rosemary.
Thinly slice the steak and transfer to a platter along with the radishes. Sprinkle with the remaining parsley and serve with the remaining butter.