01 - Preheat oven to 400°F. Combine diced butternut squash with 1 tablespoon olive oil, salt, and pepper. Spread evenly on a roasting tray and roast for 20 to 25 minutes until tender and caramelized. Lower oven temperature to 350°F.
02 - Meanwhile, heat remaining olive oil in a large skillet over medium heat. Add finely chopped onion and cook until translucent, 3 to 4 minutes. Stir in minced garlic and sauté an additional 30 seconds.
03 - Add Italian sausage to the skillet. Cook, breaking up the meat with a spoon, until browned and fully cooked. Incorporate dried oregano, sage, and ground nutmeg, then remove skillet from heat.
04 - Using a fork, lightly mash roasted butternut squash, leaving some pieces chunky for texture.
05 - In a mixing bowl, combine ricotta cheese with the large egg, half the grated Parmesan, salt, and black pepper until smooth.
06 - Grease a 9 by 13-inch baking dish lightly. Spread 0.5 cup tomato sauce on the base. Arrange 3 to 4 lasagna noodles over the sauce.
07 - Spoon one-third of the ricotta mixture over noodles. Top with one-third sausage, one-third roasted squash, a handful of mozzarella, and approximately 0.33 cup tomato sauce.
08 - Repeat the layering process two more times with remaining ingredients: noodles, ricotta, sausage, squash, mozzarella, and sauce.
09 - Place the final layer of noodles on top, spread remaining tomato sauce evenly, and sprinkle with the remainder of mozzarella and Parmesan.
10 - Cover the dish with aluminum foil sprayed with oil. Bake at 350°F for 30 minutes. Remove foil and bake for another 15 to 20 minutes until cheese is golden and bubbling.
11 - Allow lasagna to cool for 10 to 15 minutes before slicing. Garnish with fresh basil leaves if desired.