Preheat the oven to 325 degrees F. Season the pork with salt and pepper. Heat the oil in a Dutch oven or ovenproof pot set over medium-high. Add the pork, brown on all sides, and then set on a plate. Drain all but 1 Tbsp. of the fat from the pot. Add the leek and garlic and cook 3 minutes. Add the stock/flour mixture, sage and mustard and bring to a simmer.
Set the roast back in the pot. Cover and cook in the oven 75 minutes. Add the carrots and parsnips and cook 60 minutes more, or until the pork is very tender. Remove the pork from the pot. Slice and plate the pork. Top with the saucy vegetables and serve.