Step 1: Preheat your oven to 400°F (200°C). Toss the cubed butternut squash with olive oil, salt, and pepper on a baking sheet. Roast for 25-30 minutes until tender and slightly caramelized.
Step 2: While the squash roasts, melt butter in a large saucepan over medium heat. Whisk in the flour and cook for 1-2 minutes to create a roux. Slowly whisk in the milk, stirring constantly until the sauce thickens. Remove from heat and stir in the sage, nutmeg, and a pinch of salt.
Step 3: In a small bowl, mix the ricotta cheese with half of the Parmesan and the chopped spinach. Mash about half of the roasted butternut squash into the ricotta mixture for extra flavor.
Step 4: Reduce oven heat to 375°F (190°C). Spread a thin layer of béchamel sauce in the bottom of a 9x13 inch baking dish. Place a layer of noodles on top.
Step 5: Spread a layer of the ricotta-squash mixture over the noodles, followed by a layer of roasted squash cubes and a sprinkle of mozzarella. Repeat the layers until all ingredients are used, ending with a layer of béchamel and a generous topping of mozzarella and the remaining Parmesan.
Step 6: Cover with foil and bake for 30 minutes. Remove the foil and bake for another 15-20 minutes until the top is bubbly and golden brown. Let it rest for 10 minutes before slicing.