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1
Make the filling: In a small saucepan bring water to a boil and remove pan from heat.
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2
Stir in the porcini.
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3
Soak the porcini 20 minutes.
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4
Lift out the porcini, squeezing out excess liquid, and reserve soaking liquid.
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5
In a sieve, rinse the porcini to remove any grit and pat dry.
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6
Chop the porcini and transfer to a large bowl.
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7
Simmer reserved soaking liquid until reduced about 1/4 cup.
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8
Pour the liquid through a sieve lined with dampened paper towels into the bowl with the porcini.
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9
Quarter white mushrooms and in a food processor pulse 3 batches until finely chopped.
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10
Cut zucchini into 1/4-inch dice.
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11
Chop onion and mince garlic.
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12
In a 12-inch heavy skillet heat 1 tablespoon butter over moderate heat until foam subsides and cook 1/3 white mushrooms with 2 tablespoons sherry, stirring, until liquid mushrooms give off is evaporated and they begin to brown.
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13
Add mushroom mixture to the porcini.
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14
Cook remaining mushrooms in 2 batches in butter with remaining sherry in the same manner and add to the porcini mixture.
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15
In skillet cook onion in remaining tablesppon butter, stirring, until softened.
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16
Stir in garlic, thyme, salt, and pepper and cook, stirring, until fragrant, about 30 seconds.
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17
Stir onion inot mushroom mixture until combined.
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18
Filling may be made one day ahead and chilled, covered.
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19
Make the sauce: In a 3 quart heavy saucepan melt butter over moderately low heat and whisk in flour.
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20
Coo roux, whisking 3 minutes and whisk in milk.
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21
Bring sauce to a boil, whisking constantly and simmer, whisking occasionally, 3 minutes.
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22
Stir in Parmesan (4 1/2 ounces), mustard, and salt.
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23
Remove pan from heat and cover surface of sauce with waxed paper.
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24
Sauce may be made one day ahead and chilled, covered.
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25
Bring sauce to room temperature before proceeding,.
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26
Preheat oven to 375 degrees and butter a 3 quart baking dish (9x13 inch).
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27
Assemble the Lasagna: Spread 1 1/4 cups sauce in baking dish and cover with 3 pasta sheets, making sure they don't touch each other.
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28
Spread 1/3 filling over pasta sheets in the dish and top with 3 more pasta sheets, gently pressing down layers to remove air pockets.
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29
Top pasta sheets with 1/3 mozzarella.
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30
Continue layering in the same manner with sauce, pasta sheets, filling and mozzarella, ending with mozzarella (dish will be filled to the rim).
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31
Spread remaining sauce over top and sprinkle with Parmesan (1 1/2 ounces).
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32
On a foil lined baking sheeting, bake lasagna in middle of oven until bubbling and golden, about 45 minutes.
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33
Let lasagna stand 20 minutes.
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34
Lasagna my be made one day ahead and chilled, covered.
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35
Bring lasgana to room temperature and reheat before serving.
