01 - Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with oil or nonstick spray.
02 - Heat olive oil in a large skillet over medium heat. Sauté onion for 3–4 minutes until translucent. Add garlic, cook for 1 minute. Stir in spinach until wilted, then remove from heat and allow to cool slightly.
03 - Combine ricotta cheese, egg, ¼ cup Parmesan, nutmeg, oregano, thyme, salt, and black pepper in a medium mixing bowl. Add the sautéed vegetable mixture and stir until fully incorporated.
04 - In a separate bowl, mix pumpkin purée with milk until smooth and combined.
05 - Spread one-third of crushed tomatoes evenly over the base of the prepared baking dish. Arrange 3 lasagna noodles atop the sauce.
06 - Spread half the ricotta-spinach mixture over noodles. Evenly distribute half the pumpkin mixture on top, and sprinkle with one-third of the mozzarella cheese.
07 - Add one-third of the remaining crushed tomatoes, layer with 3 more noodles, then top with the remaining ricotta-spinach mixture and pumpkin mixture. Add another third of the mozzarella.
08 - Top with final 3 noodles, remaining crushed tomatoes, and finish with the rest of the mozzarella and Parmesan cheese.
09 - Cover dish tightly with aluminum foil. Bake for 35 minutes.
10 - Remove foil and continue baking for 15 minutes, until the cheese topping is golden and bubbling.
11 - Remove from oven and let stand for 10–15 minutes before slicing and serving.