Preheat the oven to 350 degrees F.
Bring a pot of salted water to a boil and cook the lasagna noodles according to the directions, around 8 or 9 minutes. You will need 12 lasagna noodles for this recipe. I suggest make the entire box, in case some break or rip while cooking. With any leftover lasagna noodles, you can tear them apart and use like regular pasta with sauce later in the week.
Once the lasagna noodles are cooked, immediately lay them out in a single layer on a kitchen towel or parchment sprayed with nonstick spray.
In a large bowl, whisk together the ricotta, pumpkin, parmesan, egg, a big pinch of salt and pepper (½ teaspoon or so, each) and the dried sage. Whisk until combined. Place 3 tablespoons of the mixture on each lasagna noodle. I like to place the 3 tablespoons on all the noodles before spreading it out. If any look sparse, I use any leftovers in the bowl to plump up the filling on those ones.
Spread the ricotta filling out all over the noodles. Take 1 ½ cups of the grated mozzarella and sprinkle it evenly over the filling on each lasagna noodle. Keep the remaining cheese for the top.
Roll up each lasagna noodle tightly from the end.
Heat the butter in a large skillet over medium heat. Once sizzling, whisk in the flour and let it cook for 2 to 3 minutes, until fragrant and golden. Slowly whisk in the milk, stirring the entire time to avoid lumps. Whisk constantly until the mixture thickens, about 3 to 4 minutes. Reduce the heat to low and stir in a pinch of salt and pepper along with the fresh nutmeg. Taste the sauce and if needed, add more salt and pepper.
Ladle ½ cup or so of sauce in the bottom of a 9x13 inch baking dish. Alternately, you could use a 12-inch oven safe skillet too.
Place the lasagna roll ups seam-side down in the dish. Pour the remaining sauce over top of the rollups. Sprinkle the remaining mozzarella on top along with a sprinkle of parmesan cheese. Bake for 25 to 30 minutes, until golden and bubbly.
While the roll ups bake, heat the butter in a skillet over medium heat. Add in the sage leaves and cook for 1 to 2 minutes, just until crispy.
Remove the lasagna roll ups from the skillet and top with the sage leaves. Serve with extra parmesan cheese for sprinkling.