Soak the plank. Submerge the cedar plank(s) in cold water for 1-2 hours, placing a heavy can on top so it stays under. When ready to cook, dry thoroughly.
Bring the salmon to room temperature. Allow the salmon to sit out for 20 minutes before cooking to come to room temperature. Pat dry with paper towels, brush both sides lightly with neutral oil, then season generously with 1-2 tsp kosher salt and ¼ tsp black pepper.
Preheat the grill to medium to medium-high heat, then allow it to come up to 375°F (190°C).
Make the glaze. Combine maple syrup, Dijon mustard, whole-grain mustard, soy sauce, grated garlic, fresh thyme, lemon zest, and black pepper in a bowl, then whisk well.
Add planks to the grill. Place the plank on the hot grill grates, close the lid, then heat for a few minutes until it begins to smoke and crackle.
Coat the salmon. Carefully place the salmon fillets skin side down on the plank, then brush each generously with the maple-mustard glaze. Close the lid and cook for 12-14 minutes, basting the salmon again halfway through. The cook time will depend on the thickness of the salmon (thinner types of salmon, like wild Coho salmon, will cook in 8-10 minutes). Aim for an internal temperature of 120-125°F (medium-rare); carry-over heat will raise the temperature slightly higher.
Rest, then serve. Using tongs or a large grill spatula, carefully slide the whole plank onto a rimmed sheet tray (this simply makes it easier to carry); rest for 5 minutes before serving with thinly sliced chives.