Place the cedar plank in a container. Pour 33 oz or 1 litre (4 cups) of beer in the container. Place a weight on the plank to keep it submerged. Let soak for 30 minutes to 2 hours in a cool place.
Meanwhile, in a bowl, combine the onion and maple syrup with the rest of the beer.
Place the salmon filet in a dish with high edges. Pour the maple preparation in the dish. Cover and marinate for 30 minutes to 2 hours in a cool place.
At cooking time, preheat the barbecue at medium heat.
Drain the salmon filet over the saucepan to recover the marinade.
Bring the marinade to a boil and simmer for 20 to 25 minutes until syrupy.
Drain the cedar plank and place the salmon filet on top. Add salt and pepper. Baste the salmon with the marinade.
Place the cedar plank on the hot barbecue grill. Cover and cook for 18 to 20 minutes, basting the salmon often with the remaining marinade.