Butternut Squash and Hazelnut Lasagne

For squash filling

1 large onion, chopped
3 tablespoons unsalted butter
1 – 3 lb butternut squash
1 teaspoon minced garlic
1 teaspoon salt
1/4 teaspoon pepper (recipe specifies white pepper, but I used freshly ground black)
2 tablespoons chopped fresh flat-leaf parsley
4 teaspoons chopped fresh sage
1 cup hazelnuts , toasted and coarsely chopped

For sauce

1 teaspoon minced garlic
3 tablespoons unsalted butter
5 tablespoons flour
5 cups milk
1 bay leaf
1 teaspoon salt
1/8 teaspoon pepper

For assembling lasagne

1/2 lb fresh mozzarella, coarsely grated (2 cups)
1 cup finely grated Parmigiano-Reggiano
no-boil lasagne sheets
1. Deal with the Squash:

Cut in half lengthwise and scoop out seeds (discard). Set in a baking dish, cut-side down. Add an inch or two of water to the bottom of the pan. Microwave until flesh is tender enough to scoop out with a spoon, for me it was about 15 minutes. Scoop out flesh, cut into 1/2-inch pieces.

2. Make filling:
Cook onion in butter in a deep heavy skillet over moderate heat, stirring occasionally, until golden, about 10 minutes. Add garlic, salt, and pepper and cook for just a minute or two, for the garlic to cook a little. Remove from heat and stir in squash, parsley, sage, and nuts. Cool filling.

3. Make sauce:
Cook garlic in butter in a heavy saucepan over moderately low heat, stirring, 1 minute. Whisk in flour and cook roux, whisking, 3 minutes. Add milk in a stream, whisking. Add bay leaf and bring to a boil, whisking constantly, then reduce heat and simmer, whisking occasionally, 10 minutes. Whisk in salt and white pepper and remove from heat. Discard bay leaf. (Cover surface of sauce with wax paper if not using immediately.)

4. Assemble lasagne:
Preheat oven to 425°F.

Toss cheeses together. Spread 1/2 cup sauce in a buttered 13- by 9- by 2-inch glass baking dish (or other shallow 3-quart baking dish) and cover with 3 pasta sheets, leaving spaces between sheets. Spread with 2/3 cup sauce and one third of filling, then sprinkle with a heaping 1/2 cup cheese. Repeat layering 2 more times, beginning with pasta sheets and ending with cheese. Top with remaining 3 pasta sheets, remaining sauce, and remaining cheese.

Tightly cover baking dish with buttered foil and bake lasagne in middle of oven 30 minutes. Remove foil and bake until golden and bubbling, 10 to 15 minutes more. Let lasagne stand 15 to 20 minutes before serving.