In a large Dutch oven or heavy bottom saucepan, heat the olive oil on medium-low. Add onion to the pan with a pinch of salt and cook for 3-5 minutes until soft. Add the cauliflower and turn up the heat to medium. Cook for an additional 3-5 minutes, stirring, until the cauliflower has softened up. Add the stock and water, then turn the heat to high. Bring the mixture to a simmer, cover the pot and lower the heat. Simmer for 45 minutes.
Bring a large pot of salted water to a rolling boil. Prepare the pasta according to package instructions and set aside.
Carefully ladle the sauce into a blender (in batches, if necessary). Puree until smooth and silky. Return the sauce to the pot and add the milk. Season with fresh grated nutmeg, salt and pepper to taste. Stir in cheese and/or nutritional yeast if using.
Toss the sauce with the pasta and serve.
You can use ground black pepper instead of the white pepper. White pepper is only used here to keep the sauce uniform in color.