In a small bowl, combine dried plums and port wine. Let sit for about an hour to plump the plums.
Preheat oven to 450 degrees.
Place apple slices, onion, potatoes, 3 tablespoons olive oil, 1 teaspoon of salt, rosemary and 1/2 teaspoon pepper into a medium bowl. Using your hands, toss to coat evenly. Set aside.
Place pork tenderloin into a roasting pan. Season with the remaining olive oil, salt, pepper and rosemary. Transfer the apple mixture to the roasting pan. Roast for about 18-20 minutes, stirring vegetables occasionally, until the pork is cooked to medium and onions and potatoes are cooked through and lightly browned.
While pork is roasting, transfer plums and port to a small saucepan. Add the chicken broth. Heat over medium high heat to boiling. Lower the heat so the sauce is at an active simmer (some bubbling, but not a rolling boil) and reduce until liquid is about half and is syrupy. Remove from heat. Whisk in the mustard and butter until thick and glossy. Set aside.
When roast is finished cooking, remove it from the oven and transfer to a cutting board. Tent with tin foil and let the meat rest for about 5 minutes. Slice into half-inch pieces and transfer to a serving platter. Scatter the apples, potatoes and onions, and plums around the roast. Drizzle the port sauce over the roast to serve.