Immerse and soak plank in water for 1 hour; drain.
To prepare grill for indirect grilling, heat one side of grill to high heat.
Combine mustard, honey, pepper, rosemary, and garlic, stirring well with a whisk. Brush half of the mustard mixture over pork.
Place the potato in a microwave-safe bowl, and cover with plastic wrap. Microwave at high 1 minute. Add the remaining mustard mixture and juice; toss gently to coat.
Place plank on grill rack over high heat; grill 5 minutes or until lightly charred. Carefully turn plank over; move to cool side of grill. Place pork in the middle of charred side of plank; arrange potato mixture around pork in a single layer. Cover and grill for 20 minutes or until a meat thermometer registers 160° (slightly pink).