Position an oven rack in the center of the oven and preheat to 425 ° F.

Bring a large pot of salted water to a boil. Cook the lasagna noodles 1 minute longer than the package directions for al dente. The pasta should be tender enough to roll without cracking. Rinse with cold water and drain well. Lay in a single layer on a baking sheet.

Meanwhile, melt the butter in a medium saucepan over medium heat. Add the onion, garlic and whole sage leaves and cook, stirring occasionally, until the onions are softened, 6 to 7 minutes. Sprinkle the flour over the vegetables and cook, stirring, until sandy in consistency and lightly toasted, 2 to 3 minutes. Whisk in the milk, pumpkin pie spice, 1/2 cup of the pumpkin, 1 teaspoon salt and a few grinds of black pepper. Cook, stirring frequently, until the sauce is smooth and thickens to the consistency of gravy, 6 to 8 minutes. Add 2/3 cup of the Parmesan and cook, stirring, until melted, 1 to 2 minutes. Remove the sage leaves and discard; allow the sauce to cool slightly, about 5 minutes.

Beat the egg in a large bowl then stir in the ricotta, 2 cups of the mozzarella, the remaining pumpkin, chopped sage, crushed red pepper flakes, 1 teaspoon salt and a few grinds of black pepper. Brush a 13-by-9-inch baking dish with the olive oil. Spread 1 cup of the sauce onto the bottom of the dish.

Lay half of the cooked lasagna noodles on a clean work surface and spread 2 heaping tablespoons of the ricotta mixture evenly over each. Starting with a short end, roll each noodle up and transfer to the prepared baking dish seam-side down. Repeat with the remaining noodles and ricotta mixture. Spoon the remaining sauce over the lasagna rolls and sprinkle with the remaining 1 cup mozzarella and 1/3 cup Parmesan.

Cover with foil and bake until the rolls are heated through and the sauce is bubbling, about 20 minutes. Remove the foil and bake until the cheese is browned on top, 18 to 20 minutes. Garnish with the torn sage leaves, if using, and let stand for 5 to 10 minutes before serving.