1. Cut cauliflower into florets and combine in a large pot with cubed parsnips and rutabaga. Cover with water and bring to a boil. Cook until all vegetables are very tender, 20 to 25 minutes. Remove from heat and drain.
2. Add the butter, heavy cream, dill, sea salt and white pepper to the hot cauliflower mixture and mash with a fork or potato masher until everything is well combined.