01 - Preheat oven to 400°F. Toss diced butternut squash with 1 tablespoon olive oil, kosher salt, and black pepper. Arrange evenly on a roasting tray and roast for 20 to 25 minutes until tender and caramelized. Reduce oven to 350°F.
02 - While squash roasts, heat the remaining olive oil in a large skillet over medium heat. Sauté chopped onion for 3 to 4 minutes until translucent. Add minced garlic and cook for 30 seconds. Add Italian sausage and cook, crumbling with a spoon, until browned and cooked through. Stir in dried oregano, dried sage or thyme, and ground nutmeg. Remove from heat.
03 - Lightly mash the roasted butternut squash with a fork, leaving some pieces chunky for texture.
04 - In a mixing bowl, combine ricotta cheese with the egg, half of the grated Parmesan, kosher salt, and black pepper. Mix until smooth.
05 - Grease a 9x13-inch baking dish with a thin layer of oil. Spread 1/2 cup tomato sauce over the bottom. Arrange 3 to 4 lasagna noodles to cover the sauce.
06 - Spread one-third of the ricotta mixture over noodles. Add one-third of the sausage mixture, one-third of the mashed squash, a handful of mozzarella, and approximately 1/3 cup tomato sauce.
07 - Repeat the process two more times: layer noodles, ricotta, sausage, squash, mozzarella, and tomato sauce.
08 - Finish with a final layer of noodles and the remaining tomato sauce. Top with the rest of the mozzarella and Parmesan cheese.
09 - Cover the dish tightly with aluminum foil sprayed lightly with oil. Bake at 350°F for 30 minutes. Remove foil and continue baking for 15 to 20 minutes until the cheese topping is golden and bubbling.
10 - Allow to cool for 10 to 15 minutes before slicing. Garnish with fresh basil if desired and serve warm.