Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish with butter or oil.

Cook the lasagna noodles in a large pot of boiling salted water according to the package directions. Drain and lay them flat on parchment paper to prevent sticking.

In a large skillet, melt the butter over medium heat. Add the diced onion and cook until soft and translucent, about 5 minutes. Stir in the garlic and cook for another minute until fragrant.

Stir the pumpkin puree, vegetable broth, heavy cream, sage, nutmeg, salt, and pepper into the skillet. Let it simmer for 5 minutes, then remove from heat.

In a medium bowl, combine the ricotta cheese, egg, 1 cup of mozzarella, 1/2 cup of parmesan, and the chopped spinach. Mix well until smooth.

Spread about 1/2 cup of the pumpkin sauce on the bottom of the prepared baking dish.

Place a layer of three noodles over the sauce. Spread a portion of the ricotta mixture over the noodles, then top with more pumpkin sauce.

Repeat the layers until all ingredients are used, ending with a layer of pumpkin sauce on top.

Sprinkle the remaining mozzarella and parmesan cheese over the top of the lasagna.

Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for another 15 minutes, or until the cheese is golden and bubbling.

Let the lasagna rest for 10 to 15 minutes before slicing and serving.