Preheat the oven to 350 degrees F. Place the pork, fat side up, in a roasting pan. Brush the top and sides with the mustard; sprinkle with rosemary, salt and pepper. Roast for 50 to 55 minutes, or until the centre of the roast reaches 160 degrees F on an instant read meat thermometer. Transfer the roast to a plate, tent with foil and let rest for 5 minutes. While the meat is resting, remove any fat from the pan drippings and then set the pan on the stovetop over medium-high heat and then add the 1-1/2 cups of chicken stock. Bring this to a boil while scraping the bottom of the pan to dislodge the flavourful brown bits. Slowly whisk in the flour/stock mixture. Simmer until the gravy has thickened. Season with salt and pepper to taste. Slice the roast and serve the gravy alongside.