Prepare the oven. Preheat the oven to 350°F. Lightly coat a 9×13-inch baking dish with nonstick cooking spray.
Cook the shells. Bring a large pot of salted water to a boil. Cook the jumbo shells according to the package directions until tender. Drain well, then transfer them to a baking sheet. Drizzle with olive oil and gently toss to prevent sticking. Let them cool slightly.
Make the filling. In a large bowl, combine the ricotta, pumpkin puree, Parmesan, egg, fresh sage, and nutmeg. Season with salt and freshly ground black pepper.
Stuff the shells. Spoon about 2 tablespoons of the pumpkin-ricotta mixture into each shell. Arrange the filled shells on the prepared baking dish.
Make the cream sauce. Melt the butter in a skillet over medium heat. Add the minced garlic and cook for 1–2 minutes, stirring frequently, until fragrant.
Make the roux. Whisk in the flour and cook for about 1 minute, stirring constantly, until lightly browned.
Add the dairy. Gradually whisk in the milk. Continue whisking until smooth and incorporated, about 1–2 minutes.
Finish the sauce. Stir in the half-and-half and Parmesan. Cook for another 1–2 minutes, until the sauce is slightly thickened. If it becomes too thick, gradually add a little more milk. Season with salt and pepper.
Assemble. Spread 1/2 cup of the garlic Parmesan sauce across the bottom of the baking dish. Arrange the stuffed shells in a single layer and spoon the remaining sauce over the tops.
Bake. Bake for 20–25 minutes, or until the sauce is hot, bubbly, and lightly golden around the edges.
Serve. Let the baked pasta rest for a few minutes, then serve warm with additional Parmesan or fresh sage if desired.