Soak the cedar plank in water in a roasting or other pan big enough to hold it for at least 40 minutes; leave the plank in the water until ready to grill.
Place salmon, skin-side down, in a shallow non-reactive dish. Combine the soy sauce, maple syrup, ginger, lime juice and garlic in a small non-reactive bowl and whisk to blend. Pour half of this mixture over the salmon, cover tightly with plastic wrap and refrigerate. Marinate at least 1 hour or up to 2 hours. Cover and refrigerate remaining marinade; bring to room temperature 30 minutes before using.
To cook the salmon, prepare a grill for high heat. If you have a grill that registers the heat inside the grill, the thermometer should read about 350 degrees. Remove the plank from the water and place on the hot grill. Cover the grill; heat for 5 minutes, then turn plank to the other side. The plank may be charred, which is fine -- it will add to the smoky flavor of the salmon.
Place salmon, flesh side up, on the plank. Cover grill and cook until fish flakes easily and is opaque, 15 to 20 minutes. Watch carefully as time can vary depending on the type of grill and the intensity of the heat. Remove and serve the salmon on the plank. Spoon some of the reserved marinade over each fillet. Mix together the cilantro and green onions, and garnish each serving with some of this mixture. Serve each fillet with a lime wedge to squeeze over.
Nutrition information per serving: