Preheat oven to 190°C/375°F/gas mark 5. On a baking sheet, add portobello mushrooms, stem side down. Bake for about 25-30 minutes, or until mushrooms are soft throughout. Transfer to a cutting board to cool.
In a food processor, add the cashews, nutritional yeast, vegan butter, almond/soy milk, tahini, Dijon mustard, garlic, lemon juice, sea salt, red pepper flakes, and baby spinach. Blend until smooth.
Cut up the portobello mushrooms (into ½-inch pieces) and add 2 of the mushrooms to the food processor. Blend until smooth. Add the other 2 portobello mushrooms and stir (keeping these mushrooms chunky).
To make the roasted spaghetti squash, cut the squash in half lengthwise. Using an ice cream scooper or spoon, scoop out the inside of the squash, seeds and all. Brush the halves with a little olive oil, around ½ teaspoon per half. Season with salt and pepper.
On a baking sheet lined with parchment/baking paper, place the squash halves, cut side down. Bake for about 45 minutes, and remove from oven. Flip the squash halves cut side up again, and they should be browned and steaming hot. Poke them with a fork a bit to make sure they’re fully cooked. They should be stringy and soft. If the squash is cooked and ready, start scraping the squash flesh with a fork, over and over. Continue with each half until all of the squash is in spaghetti like strands.
Transfer the squash to bowls. Add a generous amount of alfredo sauce and mix it in. Enjoy!