Step 1: Preheat your oven to 400°F (200°C). Toss the cubed butternut squash with olive oil, salt, and pepper. Spread on a baking sheet and roast for 25-30 minutes until tender and slightly caramelized.
Step 2: While the squash roasts, melt butter in a medium saucepan over medium heat. Whisk in the flour and cook for 1-2 minutes. Slowly whisk in the milk, nutmeg, and half the chopped sage. Simmer until the sauce thickens into a creamy béchamel.
Step 3: In a medium bowl, mix the ricotta cheese with the remaining sage and a handful of the Parmesan. Sauté the spinach in a pan until wilted, then squeeze out any excess liquid.
Step 4: Once the squash is done, mash about half of it with a fork for a creamier texture, leaving the rest in cubes for structure in your autumn vegetable lasagna with butternut squash.
Step 5: Reduce oven heat to 375°F (190°C). In a 9×13 inch baking dish, spread a thin layer of béchamel. Layer noodles, then ricotta mixture, squash, spinach, and more sauce. Repeat until all ingredients are used.
Step 6: Top the final layer of your autumn vegetable lasagna with butternut squash with the remaining mozzarella and Parmesan. Cover with foil and bake for 25 minutes, then remove foil and bake for another 15-20 minutes until golden and bubbly.