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1
Make 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 to about 1/4 cup.
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8
Pour liquid through a sieve lined with a dampened paper towel into bowl with the porcini.
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9
Quarter white mushrooms and in a food processor pulse in 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 same manner and add to the porcini mixture.
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15
In skillet cook zucchini in 1 tablespoon butter until tender and stir into the porcini mixture.
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16
In skillet cook onion in remaining tablespoon butter, stirring, until softened.
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17
Stir in garlic, thyme, salt, and pepper and cook, stirring, until fragrant, about 30 seconds.
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18
Stir onion into mushroom mixture until combined.
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19
Filling may be made 1 day ahead and chilled, covered.
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20
Make sauce: In a 3-quart heavy saucepan melt butter over moderately low heat and whisk in flour.
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21
Cook roux, whisking, 3 minutes and whisk in milk.
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22
Bring sauce to a boil, whisking constantly, and simmer, whisking occasionally, 3 minutes.
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23
Stir in Parmesan, mustard, and salt.
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24
Remove pan from heat and cover surface of sauce with waxed paper.
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25
Sauce may be made 1 day ahead and chilled, covered.
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26
Bring sauce to room temperature before proceeding.
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27
Preheat oven to 375 degrees F. and butter a 13 by 9-inch (3-quart) baking dish.
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28
Assemble lasagne: 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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29
Spread one third filling over pasta sheets in dish and top with 3 more pasta sheets, gently pressing down layers to remove air pockets.
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30
Top pasta sheets with 1/3 mozzarella.
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31
Continue layering in same manner with sauce, pasta sheets, filling, and mozzarella, ending with mozzarella (dish will be filled to rim).
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32
Spread remaining sauce over top and sprinkle with Parmesan.
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33
On a foil-lined large baking sheet, bake lasagne in middle of oven until bubbling and golden, about 45 minutes.
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34
Let lasagne stand 20 minutes.
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35
Lasagne may be made 1 day ahead and chilled, covered.
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36
Bring lasagne to room temperature and reheat before serving.
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37
Make sauce: In a 3-quart heavy saucepan melt butter over moderately low heat and whisk in flour.
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38
Cook roux, whisking, 3 minutes and whisk in milk.
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39
Bring sauce to a boil, whisking constantly, and simmer, whisking occasionally, 3 minutes.
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40
Stir in Parmesan, mustard, and salt.
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41
Remove pan from heat and cover surface of sauce with waxed paper.
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42
Sauce may be made 1 day ahead and chilled, covered.
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43
Bring sauce to room temperature before proceeding.
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44
Preheat oven to 375 degrees F. and butter a 13 by 9-inch (3-quart) baking dish.
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45
Assemble lasagne: 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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46
Spread one third filling over pasta sheets in dish and top with 3 more pasta sheets, gently pressing down layers to remove air pockets.
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47
Top pasta sheets with 1/3 mozzarella.
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48
Continue layering in same manner with sauce, pasta sheets, filling, and mozzarella, ending with mozzarella (dish will be filled to rim).
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49
Spread remaining sauce over top and sprinkle with Parmesan.
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50
On a foil-lined large baking sheet bake lasagne in middle of oven until bubbling and golden, about 45 minutes.
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51
Let lasagne stand 20 minutes.
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52
Lasagne may be made 1 day ahead and chilled, covered.
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53
Bring lasagne to room temperature and reheat before serving.
