In a medium baking dish, stir together maple syrup, mustard, lemon juice, salt and pepper. Add fish; brush marinade over both sides of fish. Cover and refrigerate for 30 min.
Preheat lightly oiled barbecue to high. Turn off burner on 1 side (or turn single burner to lowest setting). Discard marinade. Place fish on plank, skin side down; place plank on unlit side of barbecue. Close lid and grill until fish is cooked through, about 15 min. Place plank on serving tray; garnish with lemon wedges and dill.