Soak the cedar plank in water for at least 2 hours and up to 12.
Preheat the oven to 170 C / Gas 3.
Place the salmon on the plank, skin side down. In a small bowl, stir together the sesame oil, dark brown soft sugar, dill and sesame seeds. Stir in just enough water to make it liquid. Season with pepper and soy sauce if desired. Spoon the mixture over the salmon so it is fully coated.
Bake for 30 to 40 minutes in the preheated oven, until the fillet can be flaked with a fork.
Cedar planks for barbecuing can be purchased online or in specialty shops. Ensure your planks are untreated.
For the BBQ
Preheat a barbecue to medium heat. Place plank with fish on the grate and cover. BBQ for about 25 minutes, or until fish can be flaked with a fork.