Preheat oven to 230°C (450°F). In bowl, mix mustard, 1 tsp (5 mL) flour, oil, herbes de Provence and pepper. Brush mixture onto the roast and place in roasting pan. Roast 10 min., then reduce oven to 160°C (325°F). Cook approx. another 1 hr 5 min., then use an instant-read thermometer to check internal temperature. Check for 50°C (120˚F) for rare, 55°C (130°F) for medium-rare and 71°C (160°F) for medium. Cook additional 15 min. durations, if required, until desired doneness.
Transfer roast to a platter and tent loosely with foil to rest 15 min.
Meanwhile, make the gravy by placing the roasting pan on the stovetop over medium-high heat. Mix the beef stock with the remaining 4 tsp (20 mL) flour until smooth. Stir mixture into the pan drippings, scraping up browned bits and bringing gravy to a simmer. Carve roast and serve gravy alongside.