Submerge your cedar planks in water for at least 2 hours. You may need to put something heavy on them to keep them under water.
Add the maple syrup, tamari, lemon juice, garlic and ginger to a small pot on high heat. Bring the sauce to a boil and then turn it down to low, stirring occasionally. Cook for about 10 minutes or until the sauce has reduced in half and becomes thicker, then remove from the heat.
Turn the grill on medium high until it reaches 350°F (175°C). If possible, use indirect heat.
While the grill is heating, pat down the salmon to remove any extra moisture. Place the salmon, skin-side down, on the soaked cedar plank and brush the glaze over the whole salmon. (Optional: You can remove the skin from the salmon before placing on the plank.)
Once the grill has heated, place the cedar plank in the middle of the grill and close the lid. Cook for 12-15 minutes. It's better to slightly under cook the salmon than overcook, as it will continue cooking once removed from the grill. You can add a little more glaze to the salmon halfway through the cooking process.
Once the salmon is done, turn the grill off and use 2 tongs to transport the cedar plank to a large baking pan. Add optional toppings like green onion and serve immediately.