1 wild salmon side (about 2 1/2 pounds), skin-on and pin bones removed
1 (14- to 16-inch-wide, 7-inch-long) cedar plank
3 medium fresh lemons, cut in half horizontally
2 teaspoons olive oil
3/4 cup whole-milk plain Greek yogurt (7 ounces)
1/2 cup finely chopped fresh chives
1/3 cup fresh tarragon leaves, finely chopped
1 1/2 teaspoons kosher salt, plus more as needed
1 teaspoon freshly ground black pepper
Place the plank in a large, shallow baking dish or roasting pan, cover with water, and weigh down with a heavy can or plate to completely submerge.
Squeeze the lemon halves into the water, then drop in the squeezed halves.
Soak the plank for at least 1 1/2 hours or up to 12 hours.
Heat the broiler to high and arrange a rack in the middle.
Remove the plank from the water, pat dry with paper towels, and place on a baking sheet.
Drizzle the olive oil on the top side of the plank and set aside.
Combine all the remaining ingredients except the salmon in a medium bowl and stir until evenly mixed.
Taste and season with more salt as needed.
Rinse the wild salmon, pat dry with paper towels, and lay it skin-side down on the plank.
Season well with salt. Evenly coat the salmon with 1/2 cup of the yogurt mixture.
Place in the oven and broil, rotating once, until the flesh springs back when touched but is not hard and the yogurt is starting to brown, about 16 to 20 minutes.
Serve immediately, with the remaining yogurt mixture passed on the side.