Make the butter by adding all the ingredients to a medium bowl and mixing well. Spoon the butter mixture onto a piece of saran wrap or parchment paper. Shape into a log, roll it up and throw it in the freezer to harden. Take it out of the freezer after 15 mins and keep it in the fridge till you are ready to use it.
When you are ready to cook your salmon, heat your bbq to medium heat.
Place your salmon on the soaked wooden boards. Place the onions, dill and lemons on top.
Put the wooden boards with the salmon on the bbq and close the lid.
Make sure to keep checking to make sure the wood doesn't catch fire, but you want it to start smoking nicely.
Cook until the fish is firm to your touch or use a fork to flake it a little bit in the thickest part to check if it is done. You want it to flake, but still be really moist. It will take about 10 - 15 mins per inch of thickness of your salmon.
Remove from the bbq and serve right away with a slice of the compound butter on top.