Bring a large pot of generously salted water to a rolling boil. Add the dry pasta and cook it according to the package directions until it reaches an al dente texture.
Drain the pasta thoroughly and place it aside until the sauce is ready. When the pasta must stand for a while, it may be lightly tossed with olive oil to help keep the pieces from adhering to one another.
While the pasta cooks, place ¼ cup of plant-based butter in a large saucepan over medium-low heat. Allow the butter to melt completely.
Sprinkle the flour into the melted butter. Stir continuously with a wooden spoon and cook the roux until it develops a pale golden hue.
Add the minced garlic and chopped sage. Cook for 1 minute, stirring frequently so the aromatics do not scorch.
Gradually pour in the vegetable broth while whisking without interruption. Continue whisking to produce a smooth mixture without flour lumps.
Increase the heat to medium-high. Bring the sauce to a boil, whisking steadily, and cook until it thickens.
Remove the saucepan from the heat. Add the pumpkin purée and whisk until the sauce is completely smooth and evenly blended.
Taste the sauce, then season it with salt and freshly ground black pepper as needed.
Add the drained pasta to the prepared pumpkin-sage sauce. Toss gently until every piece is evenly covered.
Transfer the coated pasta to a 2½-quart baking dish. When the mixture appears too dry, add ¼–½ cup of vegetable broth or water and stir carefully to distribute the liquid.
Melt the remaining 2 tablespoons of vegan butter in a medium skillet over medium heat.
Add the panko breadcrumbs and stir until the crumbs are thoroughly coated with butter and have just begun to toast. Remove the skillet from the heat.
Distribute the toasted panko evenly over the pasta mixture.
Place the baking dish in an oven preheated to 375°F. Bake for 20–25 minutes, or until the breadcrumb layer is golden and the pasta is hot throughout.
Serve immediately while the sauce remains creamy and the panko topping is crisp.