1. Soak cedar grilling plank in water to cover 2 hours, keeping it immersed.
2. Prepare grill for direct-heat cooking over medium-hot charcoal (medium-high heat for gas barbeques)
3. Stir together mustard, honey, rosemary, zest, and 1/2 teaspoon each of salt and pepper. Spread mixture on flesh side of salmon and let stand at room temperature 15 minutes.
4. Put salmon on plank, skin side down. Grill, covered with lid, until salmon is just cooked through and edges are browned, 13 to 15 minutes. Let salmon stand on plank 5 minutes before serving.