To cook the pumpkin or butternut squash in the oven, use a very sharp knife to slice the pumpkin or butternut squash in half, then scoop out the seeds and stringy bits using a spoon. If the vegetable is too thick or hard to cut, pop it in the microwave for 60-90 seconds to soften. For roasting, follow step 2; for steaming, follow step 3.
To roast the pumpkin or butternut squash in the oven, preheat the oven to 425°F (or 218°C). Drizzle the pumpkin or squash halves with a bit of olive oil or avocado and season with salt and pepper. Place the halves, flesh side down, on a parchment paper-lined or aluminum foil-lined baking sheet. Roast in the preheated oven for 30-40 minutes until completely tender and lightly browned (pumpkin should take about 30 minutes; butternut squash closer to 40 minutes). Once the pumpkin or squash is cool enough to handle, scoop out the flesh and discard the skin.
To steam the pumpkin or butternut squash in an Instant Pot or pressure cooker, pour 1 cup of water into the inner cooking pot and lower a compatible steamer rack or the trivet that comes with the Instant Pot into the inner pot. Place the pumpkin or squash halves in the basket or on top of the trivet, close the lid, and pressure cook at high pressure for 12-14 minutes. Once the timer is up, perform a quick pressure release by manually releasing the steam valve. Once the pumpkin or squash is cool enough to handle, scoop out the flesh and discard the skin.
While the squash is cooking, bring a large saucepan of water to a boil and salt generously. Cook the pasta according to the box directions until al dente and drain in a colander.
Measure out 1 1/2 cups of the roasted or steamed pumpkin or butternut squash flesh. Reserve the rest for another use, such as a curry, soup, or side dish.
In a high-powered blender, add the 1 1/2 cups of the pumpkin or squash flesh. Add in the soaked and drained cashews, nutritional yeast, mustard powder, garlic powder, onion powder, paprika, nutmeg, 1 1/2 teaspoons kosher salt, black pepper to taste, miso paste, tapioca flour or cornstarch, lemon juice, and lite coconut milk. Blend until the sauce is completely smooth and creamy, about 1-2 minutes. Taste for seasonings and adjust accordingly, adding more salt as needed, more miso paste for more umami, or more lemon juice for more acidity.
Return the cooked and drained pasta back to the saucepan and add the cheese sauce. Heat over medium heat until the sauce is well combined with the pasta and warmed through. Season the mac and cheese to taste.