Heat the oil in a large saucepan over medium heat until shimmering. If using the optional shallot, add it and stir, cooking until slightly softened, about 3-4 minutes. Add the nutritional yeast, mustard, turmeric, tomato paste, garlic, and salt. Stir and cook for about 45 seconds, or until fragrant. Add the almond milk, scraping the bottom of the pan as you stir it in. Bring to a steady simmer.
Add the miso, cauliflower, and cashews to the pan, reduce the heat to medium-low, and partially cover the pan. Continue to cook, stirring occasionally, until the cauliflower is fork-tender, about 20 minutes.
Add the mixture to your blender (working in batches if needed) and blend until completely smooth. If you don't have a high-powered blender this may take a few minutes to break down the cashews. Stop to scrape down the sides as needed. Set aside when done.
Meanwhile, cook your pasta in a pot of generously-salted water. Cook until al dente, according to package instructions. Reserve 1/2 cup of the pasta cooking water, then drain the pasta (but do not rinse it), leaving it in the strainer.
Add the pureed cauliflower mac and cheese sauce to the pot, and turn the heat to medium low. Cook, stirring frequently, until gently simmering. Stir in the cooked pasta, the white vinegar, and a splash of the pasta water. Continue to cook until the sauce is slightly thickened onto the pasta. Add more of the pasta water to thin out the sauce as needed.
Season to taste with additional salt and pepper and serve.
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