Cut pumpkin in half and remove seeds (and save to roast for a tasty snack). Brush inside with olive oil and place cut side down on a parchment-lined baking sheet. Bake for 30 minutes, until soft and beginning to brown. Allow to cool to the touch and use a large spoon to scrape flesh from skin. Discard skin. Sauté shallot in vegan butter until soft and translucent.
Place soaked cashews and 1 cup water in a food processor and process until very smooth and creamy. Add pumpkin, potatoes, shallot, nutritional yeast, lemon juice, salt, mustard, and paprika and process until smooth.
Turn on broiler. In a small baking dish, toss panko breadcrumbs with melted vegan butter and chopped sage; broil for 1 minute, until crisp and golden brown.
Cook pasta in boiling water and drain, reserving a little of the cooking water. Transfer sauce to a medium saucepan and heat gently, adding a little pasta water to thin if necessary. Season to taste with salt and pepper. Pour over cooked pasta.
Spoon into bowls and sprinkle with toasted breadcrumbs and sage leaves.