Preheat oven to 350°. Unwrap the pork and put in a casserole dish or roasting pan just large enough to hold the roast comfortably.
Stir together the mustard, salt, Worcestershire, herbs and black pepper in a small bowl. Apply the herb mixture to all sides of the roast. Roast the pork, uncovered and fat cap up, for three hours.
After three hours check for fork tenderness. You want the pork's internal temperature to be 190°-200°. Ovens vary, so you may need to check a few times. Once the roast is fork tender and has reached 190°, take it out of the oven and let it rest undisturbed for 30 minutes.
After 30 minutes, remove the pork to a cutting board and pour the drippings in a measuring cup. Add enough chicken stock to equal 2 cups liquid. Melt the butter in a sauce pan over medium heat. Whisk in the flour and cook, whisking constantly for a minute or two. Add a third of the dripping liquid and whisk in well. Stir in the rest of the liquid, and cook, stirring frequently, until the gravy reaches your desired consistency. Taste for salt and pepper and adjust if necessary.
