Place the cashews in a heatproof bowl and cover them with boiling water. Let them soak for about 10 minutes, then drain well. For an even silkier sauce, you can soak them in cool water for 4 hours or overnight.
Bring a large pot of generously salted water to a boil. Cook the pasta according to the package directions until al dente. Before draining, reserve about ½ cup of the pasta water.
While the pasta cooks, add the soaked cashews, plant-based milk, vegetable broth, garlic, nutritional yeast, lemon juice, olive oil, Dijon mustard, onion powder, nutmeg, salt, and black pepper to a high-speed blender.
Blend on high for 1–2 minutes, or until the sauce is completely smooth and creamy. Scrape down the sides if needed and blend again until no pieces of cashew remain.
Pour the sauce into a large skillet and warm over medium-low heat for 3–4 minutes, stirring occasionally. Avoid bringing it to a rapid boil, as the sauce can become overly thick.
Add the drained pasta directly to the skillet and toss gently until every strand is coated in the creamy Alfredo sauce.
Add reserved pasta water a tablespoon at a time until the sauce reaches your preferred consistency. The pasta water helps loosen the sauce while allowing it to cling beautifully to the noodles.
Taste and adjust the seasoning with additional salt, black pepper, or lemon juice if needed.
Serve immediately with fresh parsley and a sprinkle of red pepper flakes, if desired.