In a small heavy saucepan simmer maple syrup, gingerroot, 3 tablespoons lemon juice, soy sauce, garlic, and salt and pepper to taste until reduced to about 1 cup, about 30 minutes, and let cool. (Maple glaze may be made 2 days ahead and chilled, covered. Bring maple glaze to room temperature before proceeding.)
Preheat oven to 350 degree. If using cedar plank, lightly oil and heat in middle of oven 15 minutes; or lightly oil a shallow baking pan large enough to hold salmon.
Arrange scallion greens in one layer on plank or in baking pan to form a bed for fish.
In another small saucepan heat half of glaze over low heat until heated through to use as a sauce. Stir in remaining tablespoon lemon juice. Remove pan from heat and keep sauce warm, covered.
Put salmon, skin side down, on scallion greens and brush with remaining glaze. Season salmon with salt and pepper and roast in middle of oven until just cooked through, about 20 minutes if using baking pan or about 35 if using plank.
Cut salmon crosswise into 6 pieces. On each of 6 plates arrange salmon and scallion greens on a bed of mashed potatoes. Drizzle salmon with warm sauce.
Accompaniment: Mustard Mashed Potatoes (recipe follows).
In a 5 quart kettle cover potatoes with cold salted water by 2 inches and simmer until tender, 35 to 45 minutes. While potatoes are simmering, in a small saucepan heat milk with butter over moderate heat until butter is melted. Remove pan from heat and keep milk mixture warm, covered.
In a colander drain potatoes and cool just until they can be handled. Peel potatoes, transferring to a large bowl. Add mustard, salt and pepper to taste, and three fourths hot milk mixture and with a potato masher mash potatoes until smooth, adding more milk mixture if necessary to make them creamy. (Potatoes may be made 1 day ahead and cooled completely before being chilled in a buttered ovenproof dish, covered. Bring potatoes to room temperature before reheating, covered.)
Recipe Courtesy of Gourmet Magazine