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1
Season salmon fillets with salt. Cover and refrigerate for 1 hour.
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2
Wipe the excess salt from the salmon fillets, and wrap the fillets with 2 layers of cheesecloth. Mix 1 cup miso, 1/4 cup sake, and mirin together in a bowl. Spread both sides of the wrapped fillets with the miso mixture. Place the salmon in an air-tight container, and refrigerate for at least 5 hours, or overnight.
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3
Heat a small saucepan of water over medium-high heat. Add the spinach, and cook until wilted, 1 to 2 minutes. Drain in a colander, and allow to cool. Place spinach in a food processor. Process until smooth, adding water if the spinach becomes too dry. Set aside.
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4
Mix the remaining 1/2 cup of miso with sugar, egg yolks, and 2 tablespoons sake in the top of a double boiler over barely simmering water, stirring frequently and scraping down the sides with a rubber spatula until thickened. Set aside to cool, then stir in the pureed spinach.
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5
Preheat the oven's broiler and set the oven rack about 6 inches from the heat source. Lightly grease a baking sheet. Unwrap the cheesecloth, and scrape off excess miso from the salmon fillets. Arrange fillets on the prepared baking sheet.
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6
Broil in the preheated oven until the salmon is golden, about 4 minutes. Flip the fillet over, then brush with the spinach sauce. Continue broiling until the salmon is cooked through and easily flaked with a fork, about 4 minutes more.
