Begin by submerging the cedar grilling plank completely in water and letting it soak for a full 2 hours. This prevents the wood from burning and allows it to smolder gently during grilling, imparting a rich, smoky flavor to the salmon. Use a heavy object to keep the plank fully submerged throughout the soaking time.

While the plank is soaking, prepare your grill for direct heat. For a charcoal grill, build a medium-hot fire and open the vents on both the bottom and the lid to maintain consistent airflow. If you're using a gas grill, preheat to medium-high heat.

In a small mixing bowl, combine the grainy mustard, honey (or maple syrup), freshly grated lemon zest, and minced rosemary. Add ½ teaspoon each of salt and freshly ground black pepper to the mixture, stirring until a thick, aromatic glaze forms.

Pat the salmon dry and place it skin side down on a work surface. Generously slather the mustard glaze across the flesh side of the fillet, spreading it evenly from edge to edge. Allow the salmon to rest at room temperature for about 15 minutes, giving the flavors a chance to meld before hitting the heat.

Once the grill is ready, place the soaked cedar plank directly on the grill grates. Carefully transfer the salmon onto the plank, skin side down. If the fillet is wider than the plank, gently fold the thinner tail end underneath to ensure it fits snugly.

Cover the grill with the lid and cook the salmon for 13 to 15 minutes, or until the flesh is opaque and just flakes with a fork. The edges should begin to caramelize and take on a rich, golden hue. Once done, let the salmon rest on the plank for 5 minutes before serving. This brief resting period allows the juices to redistribute, resulting in a moist and flavorful final dish.