Preheat the oven to 400°F. Heat the oil in an ovenproof skillet over medium-high heat. Season the beef with salt and pepper, place in the skillet and sear on all sides. Remove the skillet from the heat.
Brush the top and sides of the roast with the mustard; sprinkle with the rosemary, salt and pepper. Place the skillet in the oven and roast about 20 to 25 minutes for rare to medium-rare beef, and 25 to 28 minutes for medium-rare to medium (see Note).
Transfer the beef to a plate, tent with foil and let rest while you make the sauce.
Place the stock and flour in a small bowl and whisk to combine. Drain any excess fat from the skillet and then set on the stovetop over medium-high heat. Add the wine to the skillet and cook until reduced by half. Pour in the stock/flour mixture, bring to a simmer, and simmer until lightly thickened. Season the sauce with salt and pepper and set aside on low heat.
Thinly slice the beef, arrange it on a small serving platter and drizzle with some of the sauce. Serve the remaining sauce in a small sauceboat alongside.
Note: The best way gauge doneness is to use an instant-read meat thermometer, inserted into the centre of the thickest part of the roast. For a rare roast, remove from oven when internal temperature is 120°F to 125°F; for medium-rare, remove from oven when internal temperature is 130°F; and for medium, remove from oven when internal temperature is 140°F.