Soak plank in water to cover 1 hour; drain.
Season salmon with sea salt and pepper. Place fish on plank.
Stir together onion and next 6 ingredients. Spoon onion mixture over salmon, pressing firmly.
Cover with grill lid; grill over medium-high heat (350° to 400°) 15 to 20 minutes or until fish flakes with a fork. Squeeze lemon over fish. Garnish, if desired.