For best results, slice the onion into very thin rounds on a mandoline.
Combine seasonings and oil in a bowl and whisk. Pour into a large zip-top plastic bag with salmon and onions; seal, pressing out excess air and coating salmon with marinade. Let salmon marinate in refrigerator at least 30 minutes, or up to 3 hours, turning several times.
Soak a cedar plank in water 1 hour. Set up your grill for indirect heat and preheat to about 375°. Brush salmon with olive oil and set on plank. Arrange onions in a single layer over fish. Set plank in grill away from the heat source, cover, and cook until onions are tender and salmon is just opaque in the center, 20–30 minutes, depending on thickness. Serve on plank, surrounded by lemon wedges.