Butternut Squash Lasagna

Steps

1. Roast the squash: Preheat the oven to 425°F with a rack in the center. Line a 13×18-inch rimmed baking sheet with parchment paper. Toss the squash cubes with the olive oil, ½ teaspoon of the salt, and the black pepper, then spread in a single layer with space between the cubes. Roast until deeply caramelized at the edges and completely tender when pierced with a fork, 25 to 30 minutes, flipping once halfway through.
2. Purée the squash: Transfer the roasted squash to a food processor (or a large bowl if using an immersion blender) along with the remaining ¼ teaspoon salt and the nutmeg. Blend until completely smooth, scraping down the sides as needed — you should have about 2½ cups of thick purée. Taste and add more salt if needed. Set aside.
3. Make the béchamel: Melt the butter in a medium saucepan over medium heat. Whisk in the flour and cook, whisking constantly, until it smells nutty and turns a pale golden color, 1 to 2 minutes. Gradually whisk in the warm milk, a splash at a time at first, until the mixture is smooth, then pour in the rest. Cook, whisking often, until the sauce thickens enough to coat the back of a spoon, 5 to 7 minutes. Remove from the heat and stir in the sage, nutmeg, cayenne (if using), salt, and black pepper. Taste and adjust seasoning.
4. Make the filling: Heat the olive oil in a large skillet over medium heat. Add the onion and cook, stirring, until soft and translucent, about 4 minutes. Add the garlic and cook until fragrant, about 1 minute. Add the spinach in batches and cook, stirring, until completely wilted (or add the thawed, squeezed-dry frozen spinach and warm it through), then keep cooking until any liquid in the pan has cooked off. Transfer to a large bowl and let cool for a few minutes. Stir in the ricotta, egg, Parmesan, nutmeg, salt, and black pepper until evenly combined.
5. Assemble the lasagna: Reduce the oven to 375°F. Spread ¾ cup of the béchamel across the bottom of a 9×13-inch baking dish. Arrange a single layer of no-boil noodles over the sauce (about 3 to 4, breaking to fit as needed). Spread half of the squash purée over the noodles, then dollop and spread half of the spinach-ricotta filling on top, and sprinkle with one-third of the mozzarella. Repeat with another layer of noodles, the remaining squash purée, the remaining spinach-ricotta, and another third of the mozzarella.
6. Finish the top: Add a final layer of noodles, pour the remaining béchamel over them and spread it all the way to the edges so every noodle is coated, then sprinkle with the remaining mozzarella and the ½ cup Parmesan.
7. Bake covered: Cover the dish tightly with foil and bake until the sauce is bubbling around the edges, about 25 minutes.
8. Brown and finish: Remove the foil and continue baking until the top is golden brown and the lasagna reaches 165°F in the center on an instant-read thermometer (this ensures the egg in the ricotta is fully cooked), 15 to 20 minutes more. If the top browns before the center is up to temperature, tent loosely with foil.
9. Rest and serve: Let the lasagna rest for 15 minutes so it sets and slices cleanly. Garnish with a few fresh sage leaves (or crispy fried sage) if you like, then cut into 8 squares and serve.