Soak cedar planks in water for 2 hours if you are grilling, and 30 minutes if you are roasting in the oven. You may add sea salt to the purified water.
Rinse and dry the salmon. Set aside.
In a small bowl, combine Dijon mustard, maple syrup, thyme, garlic, ginger, pepper, and sea salt, if desired. Whisk until all ingredients are well blended.
Put marinade in shallow dish and place fillets with the pink flesh side down in the marinade. Do not cover skin with marinade. Let fillets sit in the marinade for 15 minutes at room temperature, or marinate them for up to a day in the refrigerator.
Preheat grill on high. Reduce heat to medium-low after 15 minutes.
Remove fillets from marinade. Do not pour marinade over fillets while grilling. Place salmon fillets on cedar planks, skin side down, and place planks on the grill. Cover the grill and cook for 12 to 15 minutes, or until surface fat begins to turn white.
Cooking time depends on size of fillets and preferred doneness. For roasting in the oven: Preheat oven to 325 degrees. Prepare the salmon as described above. Place cedar planks on roasting tray and roast for 12 to 15 minutes. Cooking time depends on size of fillets and preferred doneness.
Serve on cedar planks—place cedar planks directly on plates.