Roast the Squash: Preheat the oven to 400°F. Toss the cubed butternut squash with olive oil, salt, and pepper. Spread on a baking sheet and roast until tender and lightly caramelized, about 25-30 minutes.
Prepare Ricotta Mixture: In a bowl, mix together ricotta cheese, egg, nutmeg, and a pinch of salt and pepper.
Assemble Lasagna: Spread a thin layer of marinara sauce in the bottom of a 9×13-inch baking dish. Layer with 3 lasagna noodles, half the ricotta mixture, half the roasted butternut squash, half the spinach, and a third of the mozzarella cheese. Repeat the layers, then top with the final layer of noodles, the remaining marinara sauce, mozzarella, and sprinkle with Parmesan cheese.
Bake: Cover with foil and bake at 375°F (190°C) for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.
Serve: Let the lasagna sit for 10 minutes before slicing. Serve warm and enjoy.