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1
Bring a large pot of water to a boil.
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2
Cook noodles until not quite al dente, about 2 minutes less than the package directions.
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3
Drain, return the noodles to the pot and cover with cool water.
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4
For the onion filling:
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5
Meanwhile, heat 2 1/2 tablespoons oil in a large skillet over medium heat.
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6
Stir in onions and cook, stirring constantly, until golden brown and very soft, 25 to 30 minutes.
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7
(If they start to stick, stir in water 3 to 4 tablespoons at a time to prevent burning.)
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8
Stir in mushrooms and cook, stirring, until just beginning to soften, 3 to 5 minutes.
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9
Stir in wine and 1 teaspoon salt and continue cooking until most of the liquid is absorbed, 3 to 4 minutes.
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10
Remove from heat, stir in black pepper.
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11
For the spinach filling:
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12
Put spinach, ricotta, basil and 1/2 teaspoon salt in a food processor and process until smooth.
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13
For the white sauce:
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14
Heat 2 1/2 tablespoons oil in a medium saucepan over medium-high heat.
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15
Stir in flour until bubbling, about 30 tio 35 seconds.
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16
Slowly add in milk and 1/2 teaspoon salt and bring to a boil, whisking frequently.
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17
Cook, whisking, until the sauce has the consistency of thick gravy, about 1 to 2 minutes
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18
Stir in cheese and gently stir until it is melted.
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19
Remove from the heat.
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20
(The sauce will continue to thicken as it sits.)
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21
For assembling lasagna:
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22
Preheat oven to 375F.
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23
Spray a 9-by-13-inch baking pan with cooking spray.
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24
Drain the noodles and spread out on a kitchen towel.
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25
Spread 1/2 cup white sauce evenly in the prepared baking pan.
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26
Arrange a layer of noodles over the sauce.
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27
Spread half of the spinach filling evenly over the noodles and top with one-third of the onion filling.
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28
Spread another 1/2 cup white sauce evenly over the onions.
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29
Repeat with another layer of noodles, the remaining spinach filling, half the remaining onion filling and half the remaining white sauce.
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30
To finish:
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31
Top with a third layer of noodles, spread the remaining onion filling over the noodles and then spread the remaining white sauce on top.
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32
Sprinkle with walnuts and basil.
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33
Bake until hot and bubbling, 30 to 35 minutes.
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34
Allow to cool for a few minutes.
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35
Serve warm.
