Submerge untreated cedar plank in water. Soak approximately 12 hours, or overnight.
Preheat an outdoor grill for high heat. Place prepared plank on the grill, and sprinkle with coarse salt. Cover grill and heat plank 2 to 3 minutes, until dry. Adjust grill temperature for medium heat.
Rub salmon filets 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.