Prep: preheat the oven to 425F and set a 9×13″ casserole pan aside.
Blend: add the soaked and drained cashews, pumpkin puree, garlic, olive oil, nutritional yeast, nutmeg, salt, and water to a high-speed blender. Blend on high for 45-60 seconds, until the cashews completely dissolve and you are left with a smooth and creamy sauce.
Mix: add the pasta, pumpkin, and shallot to the casserole dish and mix until evenly distributed. Pour the sauce over the casserole, then use a spatula or spoon to press everything down evenly into the sauce, especially the pasta noodles.
Bake: Cover the dish tightly with aluminum foil (or parchment paper, followed by aluminum foil) and bake in the middle rack of the oven for 45 minutes.
Breadcrumbs: place the sourdough on separate baking tray and bake alongside the casserole for 10-20 minutes, until dry and golden; bake time will depend on the size of your bread. Roughly break up the bread with your hands, then transfer it to a food processor along with the chopped rosemary, salt, and pepper to taste. Pulse until a fine but crumbly texture forms. Drizzle the oil over the breadcrumbs, then pulse again until evenly distributed. Transfer the breadcrumbs to a small bowl and set aside.
Make it Crispy: after 45 minutes, remove the casserole dish from the oven and stir well to evenly distribute the sauce. Cover the pasta evenly with breadcrumbs, then return to the oven and bake for an additional 7-10 minutes. (See notes for cooking time without breadcrumbs)
Serve + Store: serve warm; leftovers can be stored in the fridge and are best eaten within 5 days