Preheat your oven to 400 F.
Prepare the squash: peel the squash with a vegetable peeler. Scoop out and discard the seeds. Chop the squash into cubes. Pour the squash cubes onto a sheet pan. Drizzle the olive oil over the squash. Add the cinnamon, nutmeg and a generous pinch of salt and pepper, to taste. Toss well with your hands to coat the squash with the oil and spices, then arrange the cubes out on the pan so that they are not over-crowded. Bake for 30-35 minutes, until the squash is soft and fragrant.
Bring a large pot of water to boil. Cook the lasagna noodles 2-3 minutes less than the time the package says, stirring occasionally. You can spray them with cooking spray to avoid sticking!
Add the spinach and entire container of ricotta cheese to a bowl. Add sage, rosemary, thyme and salt & pepper to taste. Mix well until combined and set aside.
Add the cooked squash to a food processor or blender. Add the milk, salt, and pepper to the butternut squash and blend until smooth. The consistency should be similar to marinara sauce. Taste, adjust seasonings, and blend again if needed.
Preheat your oven to 375 F (190 C) (or just turn the heat down) and now it's time to assemble! Grease the bottom of a 9x13 (22x33cm) baking dish with cooking spray. Add a thin layer of butternut squash puree to the bottom of the pan. Then add a layer of noodles (I added 3 noodles to each layer).
Top the first layer of noodles with half of the ricotta/spinach mixture. Then add 1/4 cup of mozzarella cheese, followed by 1/3 cup parmesan cheese. Top with another layer of noodles.
Add a layer of butternut squash puree on top of the noodles. (Make sure you'll have enough left for a generous amount for the last layer!). Then add 1/4 cup of mozzarella cheese, followed by 1/3 cup parmesan cheese. Top with another layer of noodles.
Top the new layer of noodles with the remaining ricotta mixture, followed by 1/4 cup mozzarella cheese and 1/3 cup parmesan cheese. Then add the last layer of noodles.
Add the remaining butternut squash puree to the last layer of noodles, followed by the rest of your mozzarella cheese. Bake, covered, for 30 minutes. You can remove the cover for the last 10 minutes of baking!