Grilling the fish in this manner will give it a smoky, woody flavor. A smaller piece of fish allows you to do it on a backyard barbecue.
Place a nail on the long, thin edge of the plank, 3 inches from the end. Hammer it halfway into the plank. Continue, hammering nails along the length of the plank at 2-inch intervals. Repeat on the opposite edge, with the first nail 4 inches from the end.
Combine salt, pepper, and dry mustard in a small bowl and set aside.
Brush one side of the plank with 1 tablespoon olive oil and arrange 10 dill sprigs on it. Place the salmon, skin side down, over the dill. Brush the salmon with remaining 2 tablespoons olive oil and rub mustard mixture over the entire surface. Arrange remaining 10 dill sprigs over salmon.
Wrap one end of braided picture wire around the head of the first nail hammered. Pull the wire across the top of the salmon to the first nail on the opposite side; wrap the wire around the nail head, keeping the wire taut. Continue lacing until the wire has been fastened to all nails and salmon is secure.
Stack bricks (long sides facing each other) in two columns 3 feet apart. Tne column should be 6 bricks and the other 5. Build a medium-hot wood fire between them.
Center the plank, fish side down, over the fire with the salmon's tail closest to the 6-brick stack. Cook over fire until flesh is firm and flaky, about 30 to 35 minutes; cooking time varies depending on the heat of the fire and thickness of fish. Spritz fish and plank with water if they smolder. If the fish begins to burn before the flesh is cooked through, add another brick to each stack. If the fire is too cool, remove a brick from each stack.
When cooked, remove from fire and unwrap wire. Discard dill and cut fish directly on plank. Serve with grilled pepper relish.