Submerge untreated cedar plank in water. Soak approximately 12 hours, or overnight.
Preheat a barbecue for high heat. Place prepared plank on the barbecue, and sprinkle with coarse salt. Cover barbecue and heat plank 2 to 3 minutes, until dry. Adjust barbecue temperature for medium heat.
Rub salmon fillets with olive oil. Arrange on the plank. Top salmon with red onion, lemon slices and black peppercorns.
Cook salmon, covered, 10 to 12 minutes, or until opaque and easily flaked with a fork.
Cedar planks specifically for culinary use are available online and in specialty shops. Be sure to purchase untreated, preservative-free wood planks.