Season the steak with salt and pepper to taste. Heat the olive oil in a frypan over medium-high heat. Cook the steak until nicely browned on the outside and nearly cooked to your desired degree of doneness on the inside, 3 to 4 minutes per side for medium-rare. Remove the steak from the pan and tent loosely with foil to keep warm.
Wipe the oil from the frypan and pour in the red wine; simmer until reduced by half. Whisk the flour into the beef stock then stir into the simmering red wine along with the rosemary and Dijon mustard. Return to a boil; cook, stirring constantly until the sauce reduces slightly and thickens. Keep warm over low heat.
Cut the meat from the bone, then slice into 1cm thick pieces. Serve with the rosemary sauce.