Season steaks all over with salt (at least 1 teaspoon salt per steak).
Set steaks on a wire rack set inside a rimmed baking sheet. If you have the time, refrigerate steaks, uncovered, at least 2 hours or up to 12 hours. If you don’t have time, skip this step and move on to step 3.
Heat oven to 200°F (93ºC). (Or as low as your oven goes. If it’s higher than 200ºF the process will take longer).
Cook steaks in oven until an instant-read thermometer registers 105°F (41°C) for rare, 115°F (46°C) for medium-rare, 125°F (52°C) for medium, or 135°F (57°C) for medium-well. About 20 minutes for rare and up to 40 minutes for medium-well.
Keep an eye on internal temperature—a lot of factors can impact cooking time.
About 5 minutes before steaks come out of oven, heat a large cast-iron skillet over high heat until nearly smoking. Once steaks are removed from oven, add 2 tablespoons butter and 2 tablespoons oil to hot skillet; swirling to melt butter and coat skillet.
Add steaks, garlic, and rosemary (if using) to skillet and cook until each side is well-browned, about 1 ½ minutes per side.
Using tongs, hold steaks upright on their sides to sear.
Transfer steaks to a cutting board and let rest 2 minutes. Slice and enjoy immediately.