Preheat the oven to 350F. Season the steak all over with salt and pepper.
Over high heat, melt the oil and butter in a large oven-proof sauté pan. Once the butter's foam disappears, add the steak to the mixture and cook until it obtains a rich brown crust on one side, approximately 3 minutes. Turn it over and brown for 3 minutes on the other side.
Place in the oven, moving the oven rack to the middle rack, and cook until the meat reaches an internal temperature of 125F for medium rare and 140F for medium.
Loosely cover the meat with foil and let it rest for 5 minutes before slicing. For best slicing, use a serrated knife and cut very thin slices against the grain. Drizzle any pan juices over the meat before serving.Enjoy Pan Roasted Flank Steak!