Do Ahead:

The maple glaze may be made 2 days ahead and chilled, covered.

Bring to room temperature before proceeding.

Salmon:

The potatoes may be made 1 day ahead. Chill in a buttered ovenproof dish, covered. Bring to room temperature before reheating, covered.

In a small heavy saucepan, simmer the maple syrup, ginger, 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.

Preheat the oven to 350° F. If using a cedar plank, lightly oil and heat the plank in the middle of the oven 15 minutes; or lightly oil a shallow baking pan large enough to hold the salmon.

Arrange the onion greens in one layer on the plank or in the baking pan to form a bed for the fish.

In another small saucepan, heat half of glaze over low heat until heated through to use as a sauce. Stir in the remaining tablespoon lemon juice. Remove the pan from the heat and keep sauce warm, covered.

Put the salmon, skin side down, atop the onion green and brush with the remaining glaze. Season the salmon with salt and pepper and roast in the middle of the oven until just cooked through, about 20 minutes if using a baking pan or about 35 minutes if using a plank.

Cut the salmon crosswise into 6 pieces. On each plate, arrange the salmon and onion greens on a bed of mashed potatoes. Drizzle the salmon with warm sauce.

Mustard Mashed Potatoes

In a 5-quart kettle or pot, cover the potatoes with cold salted water by 2 inches and simmer until tender, 35 to 45 minutes. While the potatoes are simmering, in a small saucepan, heat the milk with the butter over moderate heat until the butter is melted. Remove from the heat and keep the milk mixture warm, covered.

In a colander, drain the potatoes and cool just until they can be handeled. Peel the potatoes, transferring to a large bowl. Add the mustard, salt, and pepper to taste, and ¾s of the hot milk mixture. With a potato masher, mash until smooth, adding more milk mixture if necessary to make the potatoes creamy.