Salt and pepper one side of the steak.
Heat your seasoned cast iron skillet to smoking hot. Add oil and swirl around to coat. Add steak seasoned-side down (place it in pan away from you so you don’t get splattered), and then salt and pepper the other side of the steak. At this point do not touch it for two minutes. Using tongs (never pierce the meat with a fork), flip the steak and add butter, garlic and parsley to the pan next to the steak. Allow the steak to cook for 2 more minutes.
Right after you flip the steak for the first time, with a spoon or small ladle, keep basting the melted butter over the steak. Baste continually for the full two minutes (tilt pan a little if you have to, to get the butter onto the spoon).
After two minutes on each side, keep flipping and basting the steak each time leaving the steak for 30 seconds before turning. Test the steak with the poke test (see note below) and remove at medium rare at about the five to six minute mark of total cooking time. Cook a minute or two longer for medium to well. A thicker steak (such as a sirloin) may take longer.
Turn off the heat and baste one more time. Leave the steak in the pan loosely covered with foil for 10 minutes and allow to rest before cutting. Baste one more time, and remove to a cutting board. Either cut the steak in half for each serving or for a nicer presentation, slice on the bias and serve slices.
Discard the pan drippings or if desired, deglaze the pan with a little wine, then and stock and simmer for a few minutes to reduce the drippings. Add a tablespoon or two of butter to thicken for a nice pan sauce