Cut squash in half lengthwise. Remove seeds and place each half face down on a greased baking pan. Roast at 400° for 45 minutes or until squash is tender. Reduce oven to 375°.
While squash is in the oven, cook lasagna noodles according to package instructions.
Set noodles and squash aside to cool. Once squash is cool enough to handle, scoop flesh into a food processor and puree until smooth.
Add 1 c. milk and 1/2 tsp. nutmeg and continue blending until smooth.
In a large skillet, heat 1 Tbsp. olive oil over medium-high heat. Add 1 c. chopped onion and 8 oz. spinach; saute 5 minutes or until onion is tender. Set aside.
Beat 2 eggs in a large mixing bowl. Add 15 oz. ricotta, 1 c. parmesan, 1/2 c. mozzarella, 1/2 tsp. salt, and 1/4 tsp. pepper.
Add spinach mixture to ricotta mixture and stir to combine.
Spread a small amount of butternut squash sauce onto the bottom of a greased 9x13 inch baking pan.
Add a layer of lasagna noodles, followed by a layer of ricotta mixture and 1/2 c. mozzarella.
Add a layer of butternut squash sauce, followed by 1/2 c. mozzarella.
Continue layering, ending with a layer of mozzarella on top.
Bake, covered, for 30 minutes. Uncover and bake for an additional 10 minutes.