Preheat oven to 375˚F.
Slice squash in half lengthwise and place onto a baking sheet, cut side down and roast for about 1 hour or until squash becomes fork tender.
Allow squash to cool for about 15 minutes, before scraping the flesh out with a fork, making sure to keep the peels intact. Place squash innards in a mixing bowl, lightly season with salt and pepper and set aside.
Add oil to a small skillet and place over medium heat. Add shallots and garlic and sauté for 3 to 4 minutes.
Remove from heat and transfer to a blender. Drain cashews and add to blender along with 1 ½ cups water, pumpkin puree and nutritional yeast. Blend until mixture is smooth. Season with salt and pepper and add more water, if needed. Pulse a few times.
Add spaghetti squash to the skillet and toss together with alfredo sauce and half of sliced green onion. Adjust seasonings and stuff hollowed squash peels with squash mixture.
Top each stuffed squash with a sprinkle of vegan Parmesan.
Place squash until a broiler and broil for about 2 minutes or until the tops of each squash brown and bubble.
Top each stuffed squash with more vegan Parmesan and thinly sliced green onion and serve.