Generously season steak with salt & pepper. Set steak on a wire rack set in a rimmed baking sheet & refrigerate, uncovered, overnight to dry out the exterior. Optional but does get a better sear.
Preheat oven anywhere between 200 - 275°F; if your oven goes lower, you can set it to an even lower temperature, though it will take longer to cook.
Place steak in the oven & cook until an instant-read thermometer registers 105°F (41°C) for rare, 115°F for medium-rare, 125°F for medium, or 135°F for medium-well. This will take about 20 minutes for rare steak & up to about 40 minutes for medium-well; cooking time can vary dramatically depending on many factors, so check often.
Just before steak come out of the oven, add 1 tablespoon oil to a skillet & heat over high until smoking. Add steak & butter to skillet & cook until each side is well browned, about 45 seconds per side. Using tongs, hold steak sideways to sear edges.
Serve right away; there's no need to let reverse-seared steaks rest.
