Preparing the cedar plank takes a bit of preliminary effort, but it's a long-term investment. The plank can be rinsed and reused for many years, and the wood flavor improves with age.
Heat a gas or charcoal grill to medium hot. Stack bricks (long sides facing each other) in two columns, one on each side of the grill - one column should be 2 bricks, the other 3.
Combine salt, pepper, and mustard in a bowl; set aside. Brush 2 teaspoons oil on plank. Arrange 6 dill sprigs on plank; place salmon, skin side down, over dill. Brush with remaining 2 teaspoons oil, rub with salt mixture, and top with remaining 4 dill sprigs.
Wrap wire around one end of plank several times to secure it, then wrap around plank and salmon. When you reach the other end, wrap again to secure wire.
Place the plank, fish side down, on the stacks of bricks with thinnest end of fillet closest to taller stack. (Alternatively, on a kettle-style grill, you can remove rack and lay plank directly across the grill. Make sure there is a clearance of at least 5 inches between the charcoal and salmon.) Cook until flesh is firm and flaky, about 20 to 25 minutes. Spritz plank and fish with water if they smolder. If salmon cooks too quickly, add a brick to stacks. Lower if grill is too cool.
Remove from heat and discard wire and dill. Cut the salmon on plank and serve with grilled-pepper relish.