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Soak the cedar plank in cold water for 2 hours; the cedar plank should be completely immersed in water, so weigh it down (a heavy bowl works well).
Combine marinade ingredients in a shallow non-reactive dish large enough to hold the salmon. Place the salmon in the marinade and turn to coat; let marinate at room temperature for 30 minutes.
Preheat a grill to medium heat.
Place the cedar plank on the hot grill. Once the cedar plank starts to smoke, place the salmon fillet on top of it, skin side down. Arrange the sliced onions on top of the salmon. Pour the marinade over all. Cover the grill and cook for 15 minutes. If the plank catches on fire, it can be lightly sprayed with water. The cooking time will vary based on the thickness of the salmon; allow 10 minutes per inch of thickness. The salmon may be served on the cedar plank, but be careful not to overcook as the salmon will continue cooking while resting on the hot cedar plank.