Soak the cedar plank in water overnight or at least for 6 hours.
Preheat the BBQ on high. Wipe excess water off the cedar plank. In a small bowl combine the mayonnaise, Dijon mustard, minced garlic, lemon zest and lemon juice. Stir in the chopped herbs into the mix and divide it in half.
Season the salmon with salt and spread 1/2 of the mix on top. Reserve the other half for later. Place the salmon on the cedar plank skin side down.
When your barbecue is hot, place the plank on one side and turn that side down to low heat, leave the other side on medium-high. Close the lid. Very quickly you will hear the plank crackle and smoulder, which is what is supposed to happen.
Check your salmon after 5 minutes to make sure the plank is not on fire, letting as little smoke out as possible. It’s supposed to smoke but not burn. If you see any flames, use a spray bottle to put them out. You do want to see smoke, which will infuse your salmon with a delicious flavour.
The salmon will be done in 20-25 minutes. Once it’s done carefully pull the plank on top of a cookie sheet as it will be very hot.
Serve the salmon with the reserved herbed mayo.