1. Soak wood plank in water 1 hour; drain. Preheat grill to medium-high heat (350° to 400°).
2. Remove pin bones from salmon, if necessary. Combine mustard and next 5 ingredients in a small bowl, and brush half of mustard mixture over salmon, reserving remaining mustard mixture.
3. Grill plank 3 minutes or until grill marks appear. Remove from grill. Place salmon, skin sides down, on heated side of plank. Top with orange slices.
4. Grill planked salmon, covered with grill lid, 10 to 15 minutes or until desired degree of doneness. (Check occasionally to make sure edges of plank don't ignite.) Garnish, if desired. Serve with reserved mustard glaze.