Preheat the oven to 450°F. Brush the beef on all sides with mustard, sprinkle with rosemary, season with a generous amount of sea salt and coarsely ground black pepper.
Heat the oil and butter in a cast iron skillet large enough to hold the beef snugly. When the butter mixture is hot but not smoking, place the beef in the pan and sear on all sides, about 3 minutes per side.
Transfer the pan to the oven and roast the beef for another 15 to 20 minutes or until the meat is medium rare.
Remove the pan from the oven and allow the meat to rest on a cutting board for 5 minutes. Meanwhile, prepare the GoBio! Organic Gravy Cubes according to package instructions; transfer to a gravy boat.
Slice the beef into 1/8” thick slices, place on a serving plate and drizzle with the prepared GoBio! Organic Beef Gravy. Serve the remaining gravy on the side.