Preheat the oven to 450 degrees F. Place the roast in a small roasting pan; season with salt and pepper. Roast for 10 minutes, and then reduce the heat to 325 degrees F. Roast 60 to 65 minutes more, or until the centre of the roast is 130 degrees F internally when checked with an instant read thermometer. (This should yield a rare to medium-rare roast; if you like your beef medium in doneness, cook 10 to 15 minutes longer.) Set the roast on a plate, tent with foil, and rest 10 minutes.
Set the roasting pan on the stovetop over medium-high heat. Add the wine and reduce by half, scraping the bottom of the pan to release the tasty brown bits. Add the stock/flour mixture and bring to a simmer. Simmer until a lightly thickened sauce forms. Whisk in the mustards and cook 1 minute more. Slice the meat and serve with the sauce.