Preheat oven to 400°F. Season pork loin all over with salt and pepper and place in a roasting pan or baking dish. Coat roast completely with a thin layer of mustard. Roast for 30-45 minutes, or until an internal temperature of 145°F is reached. Melt butter in sauce pan over medium low heat, whisk in flour and continue whisking for 1-2 minutes, until lightly browned. Scrape drippings and brown pieces from the roasting pan into the sauce pan and stir until thick (if needed, add milk up to total 6 oz. of liquid). Serve pork over sauce and sprinkle with chopped parsley.