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1
Preheat the oven to 350.
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2
Butter a shallow 2-quart gratin dish.
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3
Heat the olive oil in a large skillet over moderate heat.
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4
Add the mushrooms and cook, stirring occasionally, until they are deeply golden brown, about 10 minutes.
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5
Stir in the spinach and cook until wilted, about 3 minutes.
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6
Season with salt and pepper, and transfer the mixture to a large bowl.
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7
Melt the 3 tablespoons of butter in a saucepan over moderate heat.
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8
Add the flour and cook, stirring constantly until smooth, for 1 minute.
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9
Add the milk and cook over moderately low heat, stirring occasionally, until thickened, about 10 minutes.
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10
Add about half of the cheese, stirring until melted and smooth.
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11
Season with salt and pepper, and keep the sauce warm over very low heat.
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12
Meanwhile, in a large pot of boiling well-salted water, cook the pasta until al dente.
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13
Drain well, then add to the bowl of spinach and mushrooms.
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14
Add the cheese sauce and stir everything together until well combined.
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15
Pour half of the mixture into the buttered gratin dish, sprinkle half of the reserved cheese over it, then top with the rest of the pasta mixture.
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16
Sprinkle the remaining cheese over the top.
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17
Bake until bubbling and lightly golden, about 30 minutes.
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18
Let sit for about 5 minutes before serving.
