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1
Make mushroom ragu: melt 2 tablespoons butter in a big skillet over medium heat; add in mushrooms; saute for 4 minutes, until they give off their liquid and it reduces to a glaze.
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2
Stir in the chopped spinach, tarragon, and nutmeg; cook/stir for 1 minute; remove from heat, cover, and set aside.
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3
Make white wine veloute: put cut up 8 tablespoons butter and onion in a large, cool saucepan; set over medium-low heat; as the butter melts, stir often so that the onion is coated.
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4
Decrease heat to low and cook until the onion is very soft, almost caramelized, and golden, stirring often, about 12 minutes.
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5
Cover and decrease heat even further, and cook for another 5 minutes, stirring once or twice.
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6
Meanwhile, position the oven rack in the center of the oven; preheat to 400u00b0.
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7
Take the lid off the pan and sprinkle with flour evenly over the onion.
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8
Whisking constantly, cook just so the flour loses its raw taste and forms a paste with the liquid in the pan, about 20 seconds (do not brown).
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9
Whisk in the broth and milk in a slow, steady stream; increase heat to medium-low and continue cooking, whisking constantly, until the sauce starts to bubble and gets noticeably thicker, about 2 minutes.
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10
Whisk in the wine and continue simmering, whisking all the while, until thickened like a heavy cream sauce, about 2 minutes.
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11
Set the pan aside off the heat and give the mixture one more good whisking.
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12
Layer the casserole in a 13x9 inch baking dish as follows: 1/2 cup white wine veloute, spread across the bottom of the dish; 3 no-boil noodles, 2 cups white wine veloute, spread over the noodles with a rubber spatula; half the mushroom ragu, spread gently over the casserole, 1/3 of the grated cheese, 3 no-boil noodles, 2 cups white wine veloute; the remaining mushroom ragus; half the remaining cheese, the remaining noodles, the rest of the white wine veloute; and finally the rest of the grated cheese.
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13
Bake, uncovered, until bubbly and set, about 40 minutes.
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14
Let stand at room temperature for 5-10 minutes before serving.
