Submerge the cedar planks in water, placing a weight on top of them, and soak for 1 to 4 hours.
To prepare the buttermilk marinade, add the buttermilk, ginger, garlic, green onions, sliced lemon, and red pepper flakes to a large freezer bag. Seal and give it a good shake.
Add salmon to the buttermilk marinade and let marinate in the refrigerator for at least 2 hours or up to overnight.
Preheat grill on high for 10 minutes. Add the soaked cedar planks to the hot grill and preheat for another 3-4 minutes, then lower the temperature to medium and cover the grill.
While your grill is heating, remove the salmon filets from the marinade and pat dry with paper towels. Season the salmon with salt and pepper. Slather each filet with 1-2 tablespoons of teriyaki sauce. In a small bowl, combine the ground ginger and brown sugar, then sprinkle 1 tablespoon of the sugar mixture over the top.
Place the seasoned salmon filets onto the preheated planks and cook for 12-15 minutes, or until they reach an internal temperature of 145 degrees and flake easily when a fork is inserted into the center of the thickest part.
Remove from the grill and brush with more teriyaki sauce, if desired, then let rest for 5 minutes.
To serve, top with sesame seeds and thinly sliced green scallions. Enjoy!