How-to Use Planks While Grilling
Soak planks in water for 3 to 4 hours. This helps create an aromatic smolder and slows the burning process of the wood. Heat a grill to 375-400°. (If using charcoal, let coals burn until covered with white ash.) Place one wet plank on grill rack, cover, and let char for about 10-15 minutes.
Toss slices of onion, zucchini an sweet bell pepper in olive oil, kosher salt, coarsely ground black pepper and a pinch of two of dried thyme. Flip charred plank and place vegetables on it. Place second wet plank on rack. Grill vegetables, covered, until tender and lightly charred, about 25 minutes.
When the second plank has charred about 15 minutes, flip it, and place the fish skin down on it. Close grill cover.
Grill until fish is cooked through and reaches 135° at the thickest part, or when it flakes easily with a fork. a 6- to 8-ounce 1/2-in.-thick fillet takes about 6-8 minutes. Slide a wide metal spatula between the skin and the flesh. The skin will stick to the plank, allowing the fillet to lift easily. Discard planks after use.