Soak the wood planks in water for at least four hours. I buy furniture-grade hardwood at a specialty wood shop and cut it down for plank grilling. Not only is this more economical than buying cooking planks, I can cut longer lengths for recipes like this. Most shops will cut the wood for you if you don’t have the tools at home.
Another tip: plastic bins for storing wrapping paper work well for soaking longer planks like you’ll be using here.
Prepare the grill for indirect cooking at 400°F.
Lay one fish on each plank. Use kitchen shears to remove the pectoral fins (the ones just behind the gills). Lightly score the skin on both sides with a sharp knife in a crisscross pattern. Brush inside and out with olive oil and sprinkle generously with salt. Tuck sprigs of rosemary around each fish. This will lend a subtle aroma without overpowering the fish.
Place the planks in the indirect cooking zone, close the hood, and cook at 400°F until an instant read meat thermometer measures an internal temperature of 120°F.
Remove the planks from the grill and discard the rosemary. Combine the lemon zest and green onion and top the fish with the mixture. Sprinkle with pepper.
Serve on the planks. When cooked properly, the meat will easily separate from the bones and the skin can be cut through with a serving fork and spoon. If you have squeamish guests, you can remove the head and tail before serving. You can even use a large spatula or turner to slide off whole sides of the fish and transfer to plates or a serving platter.
Note: planks can be washed and used again.
