Soak cedar plank in water for 30-45 minutes or while preparing the remaining ingredients for barbecuing.
Combine soy sauce, orange juice and honey in a shallow dish; add salmon, skin side up. Cover and refrigerate 30 minutes.
Meanwhile, in a medium saucepan combine cranberries, finely chopped onion, orange peel and 5 spice powder. Cook over medium heat until mixture comes to a boil, stirring occasionally. Reduce heat and simmer 6-8 minutes until onions are tender and sauce thickens. Set aside to cool.
Heat grill for medium high indirect heat. Place salmon skin side down on cedar plank; place on grill. Grill for 15-20 minutes or until salmon flesh is opaque and flakes easily with a fork.
Place grilled salmon on a serving platter. Serve with relish.