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1
To make the Mushrooms: Soak porcini in boiling-hot water, stirring occasionally, until softened, about 15 minutes.
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2
Meanwhile, wash leeks.
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3
Strain porcini through a paper towel lined sieve into a bowl, reserving liquid. Squeeze liquid from mushrooms into sieve, then rinse mushrooms well to remove any grit and chop.
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4
Cook leeks in butter in a 12-inch heavy skillet over medium heat, stirring occasionally, until tender, about 8-10 minutes. Increase heat to high, then add all the mushrooms, reserved soaking liquid, and wine and boil, stirring occasionally, until mushrooms are softened, about 5 minutes.
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5
Strain through a clean sieve into a liquid measuring cup, pressing hard on and reserving mushrooms.
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6
Return liquid to skillet and boil it until it is reduced to 1/3 cup, about 5 minutes. Add cream and boil over medium-high heat, stirring occasionally until thickened, about 3 minutes. Add mushrooms, 1/2 teaspoon salt and 1/4 teaspoon pepper, and boil, stirring, 1 minute. Transfer to a 3-quart shallow baking dish.
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7
To make the macaroni: Preheat oven to 400 degrees with rack in the middle.
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8
Cook macaroni in pasta pot of boiling salted water until al dente. Drain and rinse with cold water to stop cooking, then drain again.
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9
Meanwhile, heat milk in a small heavy saucepan over medium-low heat until hot.
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10
Melt butter in a heavy medium pot over medium-low heat, then whisk in flour and cook, whisking, 3 minutes.
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11
Whisk in hot milk, Worcestershire sauce, mustard and 1/4 teaspoon pepper and boil over medium heat, whisking constantly, 2 minutes, then whisk in 3 cups cheese until melted.
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12
Remove from heat and stir in macaroni and 1 1/2 cups cheese. Spread over mushrooms.
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13
Sprinkle evenly with remaining 1/2 cup cheese. Bake until golden, about 30 minutes.
