Preheat your oven to 400 degrees F. This important step ensures that the butternut squash roasts evenly and develops a lovely golden color. Line a large baking sheet with parchment paper for easy cleanup.
Place the butternut squash cubes on the prepared baking sheet. Drizzle with olive oil and toss until well coated. Season with Kosher salt and black pepper to taste. The seasoning will enhance the natural sweetness of the squash. Roast for about 25 to 30 minutes, or until the cubes are tender and golden. Once done, remove from the oven and let cool for 5 minutes.
While the squash is roasting, bring a large pot of salted water to a boil. Add the lasagna noodles and cook according to package directions until al dente. Stir frequently to prevent sticking. After cooking, drain in a colander and rinse with cool water to stop the cooking process. Gently separate any noodles that may be sticking together.
Spread a little olive oil on a large baking sheet. Lay the cooked noodles in an even layer, turning them over to coat both sides with oil. This step is crucial to prevent them from sticking together as you layer them in the lasagna.
In a large skillet, melt butter over medium heat. Add the minced shallot and sauté for 2 to 3 minutes until softened. Then, add the spinach, cooking until wilted. Add the garlic and cook for an additional minute, until fragrant. The combination of these flavors will create a savory base for the ricotta mixture.
In a large mixing bowl, combine the sautéed spinach mixture, ricotta cheese, Parmesan cheese, egg, sage, thyme, crushed red pepper flakes, Kosher salt, and black pepper. Mix well until fully combined and set aside. This creamy filling will be a highlight of your lasagna.
Place the roasted butternut squash in a food processor. Add vegetable broth, maple syrup, and nutmeg. Process until smooth; the mixture should be thick yet creamy. This flavorful puree will be the first layer in your lasagna.
To prepare the béchamel sauce, melt butter in a large saucepan over medium heat. Once melted, add flour and whisk for one minute to cook out the raw flavor. Gradually whisk in whole milk and bring to a gentle boil. Reduce the heat to low and simmer until the sauce thickens slightly, about 3 to 5 minutes. Remove from heat and season with Kosher salt and black pepper.
Now it’s time to assemble your lasagna. Preheat the oven to 375 degrees F if you haven’t already. Grease a 9x13-inch baking dish and spread 2/3 cup of the béchamel sauce in the bottom. Begin layering: place 3 noodles on top of the sauce, then add ⅓ of the butternut squash puree, followed by ⅓ of the spinach ricotta mixture. Sprinkle with ½ cup mozzarella cheese and pour 2/3 cup of the béchamel sauce over the noodles.
Repeat the layering process two more times. On the final layer, place the remaining 3 noodles on top, followed by the remaining 1 ½ cups of mozzarella cheese and ¼ cup of Parmesan cheese. If you like, garnish with fresh sage leaves for a pop of color. Cover the baking dish tightly with aluminum foil that has been sprayed with cooking spray to prevent sticking.
Bake in the preheated oven for 40 minutes. After that, remove the foil and bake uncovered for an additional 15 minutes, or until the top is golden and bubbly. Once done, take the lasagna out of the oven and let it stand for 10 to 15 minutes before serving. This resting time allows the layers to set, making it easier to slice.
