Immerse and soak the plank in water 1 hour; drain.
To prepare grill for indirect grilling, heat one side of the grill to high heat.
Combine vinegar and the next 6 ingredients (vinegar through lemon) in a large zip-top plastic bag; seal. Shake to combine. Add fish; seal. Marinate in refrigerator 30 minutes, turning once.
Place plank on grill rack over high heat; grill 5 minutes or until lightly charred. Carefully turn plank over; move to cool side of grill. Remove fish from marinade; discard marinade. Place fish, skin side down, on charred side of plank. Cover and grill 15 minutes or until fish flakes easily when tested with a fork. Sprinkle with onions and sesame seeds.