Get ahead: You can roast the squash noodles up to 1 day in advance; reheat them just before serving.
Spiralize squash: Peel squash and cut long neck away from round bulbous part. Spiralize long neck using shredder blade. Take handfuls of spiralized squash and cut into manageable lengths with scissors.
Combine mirin and sake in a small saucepan; bring to a boil over high heat. Boil for 20 seconds, taking care not to boil off much liquid; reduce heat to low and stir in miso and brown sugar. Whisk over medium heat until sugar has dissolved. Remove from heat; whisk in sesame oil. Let cool. Transfer to a wide heatproof glass or ceramic baking dish.
Pat fish fillets dry and brush or rub both sides with marinade, then place in baking dish and turn over a few times in remaining marinade. Cover with plastic wrap and marinate for 2 to 3 hours or up to 1 day.
Preheat oven to 375°F. Line 2 baking sheets with parchment. Divide spiralized squash between baking sheets. Toss each batch with 1 1/2 tablespoons of the olive oil; season with salt. Roast for 10 minutes. Stir squash noodles and switch baking sheets back to front and top to bottom. Roast until noodles are tender and beginning to color, 5 to 10 minutes. Keep warm. Increase oven temperature to 400°F. Remove salmon from refrigerator and transfer to a plate. Add 3 tablespoons water to marinade left in baking dish and add leeks. Toss until thoroughly coated. Bake until tender, 15 to 20 minutes, stirring every 5 minutes. Remove from heat and keep warm.
Heat remaining 1 tablespoon olive oil in a large heavy skillet over medium-high heat. Place fish skin-side down in pan. Cook until surface browns and blackens in spots, about 3 minutes per side. If necessary, finish in oven until fish is opaque but still moist, about 5 minutes. Place a serving of squash noodles on each plate. Top with a fish fillet, garnish with leeks and serve.
From Spiralize This! Copyright © 2016 by Martha Rose Shulman.