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1
Heat 1 tablespoon of oil in a large saucepan over medium-high heat. Add mushrooms; cook, stirring, until they begin to brown and any moisture has evaporated. Transfer to a large plate to cool.
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2
Heat the remaining tablespoon of the oil in the same pan over medium-high heat. Add the onion; cook until soft, 3-4 minutes. Add the frozen spinach; cover. Cook 2 minutes; remove the lid. Stir the spinach. Cover; repeat until the spinach is thawed. Remove cover; cook until the moisture from the spinach has evaporated. Transfer the spinach-onion mixture to a large plate to cool.
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3
Heat milk; set aside. Melt butter in a large heavy saucepan over medium heat; stir in flour. Cook, stirring, until mixture starts to thin, 1-2 minutes. Add warm milk in a steady stream while whisking mixture. Add salt, pepper and nutmeg to taste. Heat to a boil; reduce heat to low. Simmer sauce, stirring often until sauce coats the back of a spoon, 10 minutes. Stir in ham; remove from heat. Add three-fourths of the cheese; stir until it melts. Set aside.
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4
Spoon a few tablespoons of the bechamel-cheese sauce in a 13-by-9-inch pan. Cover the sauce with a layer of lasagna sheets. Spread the spinach-onion mixture over the first layer; cover with one-third of the sauce. Add another layer of lasagna sheets; spread the mushrooms over this layer. Cover with one-third of the sauce. Add the last layer of lasagna sheets; spread the remaining sauce over. Top with the remaining grated cheese. Cover; refrigerate until baking time, up to 12 hours. Or wrap well and freeze.
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5
Heat the oven to 350 degrees. Bake lasagna, uncovered, until heated through and the cheese is golden brown, 35-45 minutes. (If frozen, the lasagna should be baked, covered with foil, 45 minutes, then uncovered and baked until heated through, 30-40 minutes.).
