Prepare the pasta according to package directions.
Place the cauliflower florets and the onion in a large pot with the vegetable broth. Bring to a boil, then reduce heat and simmer for about 15-20 minutes until cauliflower is tender.
Remove the pot from the heat and puree the vegetables with an immersion blender until very smooth.
Place the pot back on low heat and stir in the vegan buttery spread, nutritional yeast, onion powder, garlic powder, salt, white pepper, and mustard powder. Stir until smooth and incorporated. Add the cooked pasta. Stir to combine.
Serve with more salt and pepper to taste and fresh parsley if desired.