Cook the pasta: In a large pot of boiling salted water, cook the pasta noodles according to package instructions. Drain well and set aside.
Make the Cashew Cream: Bring 3 cups of water to a boil. Pour the hot water over the cashews and let soak for 5 minutes. Drain the cashews and discard the soaking water. Add them to a blender along with the 3/4 cup fresh water and apple cider vinegar. Blend until very smooth. Set aside.
In a large pot or pan, add the onions and mushrooms and turn the heat to medium-high. Stir frequently until the mushrooms start to release their moisture, and then cook until the mushrooms are tender and have shrunk in size. You really don't need to add any oil here, but you could start with a tablespoon or two of olive oil if you want. The mushrooms release a ton of oil on their own. Season with a little salt + pepper.
Stir in the garlic and cook for 1 more minute, then sprinkle the flour on top and stir it into the vegetables. Gradually pour in the vegetable broth and dijon mustard, stirring. Bring to a boil, then lower the heat and simmer for 4-5 minutes until it thickens slightly.
Now add the cashew cream and stir it in. You should have a very creamy mushroom gravy at this point. Stir in the cooked pasta and cook until heated throughout, about 1-2 minutes. Taste, add salt as needed. Sprinkle with parsley, if desired, and serve immediately.