Start by preheating your oven to 400°F (200°C). Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to the package instructions. Cook them just until al dente so they remain firm enough for stuffing later.
Heat olive oil in a skillet over medium heat. Add the diced yellow onion along with half of the chopped sage leaves and cook until the onions become soft and translucent. This step builds the base flavor for the filling.
In a mixing bowl, combine ricotta cheese, pumpkin puree, minced garlic, nutmeg, salt, pepper, and one-third cup of shredded gouda. Stir everything together until the mixture becomes smooth and creamy. Add the sautéed onion mixture and mix well.
In a saucepan, melt butter over medium heat until it begins to turn golden and develop a nutty aroma. Reduce the heat and add chopped sage leaves, allowing them to infuse the butter with flavor. Whisk in flour and gradually add heavy cream while stirring until the sauce thickens.
Spread half of the Alfredo sauce evenly across the bottom of a baking dish. Fill each cooked shell with about two tablespoons of the pumpkin mixture and arrange them neatly in the dish.
Pour the remaining Alfredo sauce over the stuffed shells. Sprinkle the rest of the shredded gouda on top to create a melty, golden layer when baked.
Place the baking dish in the oven and bake for about 20–25 minutes. The shells should be heated through, and the cheese should be bubbly and lightly golden.
Once baked, top the shells with freshly cracked black pepper and crispy fried sage leaves. This finishing touch adds both flavor and a beautiful presentation.