Preheat oven to 325ºF.
Put a roasting rack in a shallow pan. Sprinkle pork with a generous amount of salt and set in roasting pan. Roast until the thickest part of the roast registers 180ºF, about 2-2.5 hours.
Transfer to cutting board and tent with foil. Scrape the pan juices and save.
Melt 1-2 tablespoons of butter in a heavy large skillet over medium heat. Add mushrooms. Cover and cook until tender, about 10 minutes.
Pour ½ c. pan juices (decide how much fat you want to include in this - we left a good layer on top) and broth into mushrooms. Bring to a simmer. Mix 2 tablespoons butter and flour in a small bowl to form a paste. Add mustard to the paste and gradually whisk the paste into the mushroom/broth skillet. Cook until the gravy thickens, whisking occasionally, about 5 minutes. Season to taste with salt and pepper.
Carve the pork, serve with gravy, and enjoy! We made some mashed parsnips and potatoes as a side.