Preheat the oven to 375 degrees F. Slice the butternut squash in half, remove the peel and cut into half-moon shaped pieces about 1/2 inch wide.
Place the slices on two large nonstick baking sheets in a single layer (or cover regular sheet pans with foil and spray with cooking spray). Sprinkle with salt and bake for about 30-40 minutes until slices are tender. When done, remove from oven to cool. Leave the oven on.
While the squash is baking, make the Alfredo sauce (skip this step if using store-bought Alfredo sauce). In a large nonstick skillet over medium heat, add the butter and the minced garlic and cook for about 2 minutes. Add in the flour and mix well to form a paste. Gradually add the broth and use a whisk to remove any lumps. Stir in the milk and whisk to combine. Let cook for 10-12 minutes until the sauce starts to thicken. Turn heat down to medium-low and add the Parmesan cheese and salt. Cook for another 5 minutes, stirring occasionally. Remove from heat and allow to cool.
Place the thawed spinach in a double layer of paper towels and squeeze gently to remove some of the moisture. Mix the spinach with ricotta, nutmeg, and salt in a large bowl.
When the squash and Alfredo sauce are done, assemble the lasagna. First coat the bottom of a 9 in x 13 in pan with Alfredo sauce. Place the noodles side by side to cover the bottom and top with more Alfredo sauce. Then add half of the squash in a single layer. Top the squash with Alfredo sauce and another layer of noodles. Add all of the ricotta-spinach mixture. Top with another layer of noodles, then Alfredo sauce. Add the rest of the squash. Pour the rest of the Alfredo sauce on top, add the Parmesan and then the mozzarella cheese.
Cover with foil and bake for 30 minutes. Then remove the foil and bake for another 15-20 minutes until the cheese is golden brown. Rest for 15 minutes before slicing and serving.