Let steak come up to room temperature, about 30 minutes. Preheat the oven to 415°F. Pat steak dry with paper towels and season with kosher salt. Melt the butter with the oil in a cast-iron skillet set over medium-high heat. When the foam from the butter subsides, add the steak to the hot pan. Sear until nicely browned on each side, about 2 minutes per side.

Carefully transfer the hot skillet to the oven and continue cooking until steak reaches your desired internal temperature (115–125°F for rare or 125–130°F for medium rare) and offers some resistance when poked, about 4–6 minutes longer. Remove from heat and season with salt and pepper. Let rest 5–10 minutes and serve.