Prepare the Oven and Dish: Set your oven to preheat at 375°F (190°C). Lightly grease a 9 × 13-inch baking dish to prevent sticking.
Mix the Pumpkin Sauce: In a medium-sized bowl, combine the pumpkin puree, whole milk, oregano, basil, nutmeg, crushed red pepper flakes, and a pinch of salt and pepper. Whisk everything together until smooth.
Prepare the Ricotta Filling: In a separate bowl, mix the ricotta cheese with the grated garlic, chopped sage, and parsley. Season this mixture with a little salt and pepper to taste.
Start Layering the Lasagna: Spread about 1 cup of the pumpkin sauce evenly across the bottom of the greased baking dish. Lay down 3–4 lasagna sheets over the sauce, breaking them if needed to fit properly. Add half of the ricotta mixture, followed by half of the roasted red peppers, and 1 cup of shredded fontina cheese.
Continue Building Layers: Spoon another cup of pumpkin sauce over the cheese layer, then add another 3–4 lasagna noodles. Repeat the layering process with the remaining ricotta mixture, roasted red peppers, and 1 cup of fontina. Top this layer with a final cup of pumpkin sauce.
Top with Final Layers: Lay down the last set of lasagna noodles, then spread the remaining pumpkin sauce over them. Sprinkle the top with the final cup of fontina cheese, the grated Parmesan, and, if using, the pepperoni slices.
Add Sage Topping: Toss the whole sage leaves in 2 teaspoons of olive oil until lightly coated. Arrange the leaves evenly across the top of the lasagna for extra flavor and presentation.
Bake the Lasagna: Cover the dish tightly with foil and place it in the preheated oven. Bake for 45 minutes to allow the lasagna to cook through.
Finish Cooking at Higher Heat: Increase the oven temperature to 425°F (220°C). Remove the foil and bake the lasagna for another 10 minutes, or until the cheese on top is bubbling and slightly golden.
Let it Rest: Once baked, remove the lasagna from the oven and let it sit for 10 minutes to allow the layers to set.
Serve and Store Leftovers: Slice and serve warm. Any leftovers can be stored in an airtight container in the refrigerator for up to three days.