Preheat the oven to 400° F. Line a baking sheet with aluminum foil and spray with vegetable spray.
Carefully cut the butternut squash in half, vertically. Use a spoon to scoop out the seeds and discard. Place the squash cut side down on the foil and roast for 40 minutes or until the flesh is tender and the skin is dark and leathery looking.
Set aside to cool. When the squash is cool enough to handle, scoop out the flesh and transfer to a food processor. Add the salt, pepper and nutmeg and process until smooth. (Can be made up to 3 days ahead of time).
FOR THE SAGE BECHAMEL SAUCE
In a large saucepan, melt the butter over medium high heat. Add the flour, salt, pepper and nutmeg and whisk until the flour is absorbed by the butter and forms a paste. There should be no dry bits of flour remaining.
Add the milk a little at a time, whisking constantly. It will be thick at first, but as the rest of the milk is added it will thin out. Continue to whisk constantly as the mixture warms and comes to a boil. Cook for one minute, scraping the sides of the pan to prevent sticking or scorching.
Remove from the heat and add the fresh, whole sage leaves. Stir to combine and set aside to steep.
FOR THE SAGE CRUMBLE:
In a small bowl, combine the butter, flour, sugar, parmesan cheese and sage. Use the tines of a fork to mash the mixture together. Work the mixture for 2-3 minutes. After, you should be able to pinch a bit between your fingers and the crumble will stick together. Set aside.
ASSEMBLE THE BUTTERNUT SQUASH LASAGNA:
Preheat the oven to 375° F.
Remove the sage leaves from the bechamel (scraping away any sauce that's clinging to them. Discard the sage.
Add about 1/3 cup of bechamel sauce to the bottom of a one quart casserole dish and spread around in an even layer.
Top the bechamel with dry lasagna noodles, then spoon about 1/2 cup of bechamel over the noodles. Use a spatula to spread the sauce into an even layer.
Top the bechamel with 1 cup of the butternut squash purée and spread it evenly over the bechamel.
Sprinkle 1/2 cup of the grated mozzarella over the purée layer, followed by 1/4 cup parmesan cheese.
Add another layer of lasagna noodles, followed by 1/2 cup of bechamel, 1 cup of the butternut squash, 1/2 cup of mozzarella and 1/4 cup parmesan cheese.
Top with a final layer of lasagna noodles and the remainder of the sage bechamel sauce.
Sprinkle the sage crumble over the top of the lasagna and cover with aluminum foil. Bake covered for 40 minutes, then remove the foil and bake for an additional 15 minutes.
Let the lasagna rest for 15 minutes before serving.
Notes
Heat leftovers in a low oven (about 325°F) for about 15 minutes.