1. Preheat the oven to 450°F.
2. Place the butter in a medium bowl. Add the parsley, lemon juice, 1⁄2 teaspoon of the salt, and 1⁄4 teaspoon of the pepper, and stir until combined. Refrigerate while you cook the steaks.
3. Heat a large cast-iron skillet over medium-high heat.
4. Rub both sides of the steaks with the olive oil, and season evenly with the remaining 4 teaspoons salt and 2 teaspoons pepper. Place the steaks in the hot skillet and cook for 4 minutes on each side. Then transfer the skillet to the oven and roast to the desired degree of doneness, 4 to 6 minutes for medium-rare. An instant-read thermometer inserted into the thickest part of the meat should register 130°F for medium-rare, 140°F for medium.
5. Remove the skillet from the oven; let the steaks stand for 5 minutes before serving.
6. When ready to serve, top the steaks with spoonfuls of the flavored butter, to taste, or slice the steaks crosswise into 1⁄3-inch-thick slices and serve with the butter. (Any unused butter can be stored in an airtight container in the refrigerator for up to 1 week.)
Yield: 4 to 6 servings