Combine mustard, 3 tablespoons of syrup, rosemary, salt and pepper in a small bowl. Reserve 2 Tbsp of sauce and brush remaining over pork.
Microwave potatoes on high for 2 to 3 minutes until slightly tender. Place in zip top bag and toss with reserved mustard mixture.
Heat grill to 400 to 450 degrees. Sear pork on all sides for a few seconds to seal in juices and remove from grill.
Place soaked plank on grill and close lid. When plank is smoking and crackling, arrange potatoes on plank. Drizzle potatoes with remaining maple syrup and place pork on top of potatoes. Close lid and cook until pork is medium rare to rare. (Use a meat thermometer)
Remove plank from grill and tent entire contents with foil and allow to rest for ten minutes.
**If pork is done before the potatoes, remove pork and allow potatoes to cook until tender and edges are caramelized.
