In small, heavy saucepan, sauté garlic briefly in small amount of butter, until just lightly browned. Add maple syrup, soy sauce, lemon juice, salt and pepper and bring to a boil over medium heat, then reduce heat and simmer about 30 minutes, or until reduced to about one cup. Set aside and allow temperature to reduce to room temperature. Divide into two portions.
Preheat oven or BBQ to 350°F.
Lightly oil cedar plank with melted butter or olive oil, allowing oil to soak into plank. Arrange onion greens on plank, covering the surface. Place salmon, skin side down on scallion greens and brush it with 1/2 of the glaze. Season with salt and pepper to taste.
Oven: Roast in centre of oven until just cooked through – about 35 minutes
BBQ: Cook over indirect heat (burner directly under meat is low or off, with other burner providing higher heat) until just cooked through – anywhere from 20-40 minutes, depending on the BBQ
Use remaining glaze to either further garnish the salmon when removed from grill, or as a dipping side sauce.