Heat avocado oil in a large saucepan set over medium heat. Add onion, celery, butternut squash, sweet potato, rutabaga, parsnip, carrot, salt, pepper and dried oregano and sweat for about 3 to 4 minutes, until slightly softened.
Add water and bring to the boil then lower the heat, cover partly and simmer until the vegetables are tender, about 7 to 10 minutes.
Meanwhile, in a separate saucepan, heat a tablespoon of avocado oil over medium heat then add the onion, and sweat for a few minutes, until the onion becomes translucent. Add the cauliflower and water, cover and continue cooking until the cauliflower is really soft and tender, about 5 to 8 minutes.
Transfer the cauliflower, along with its cooking water, into a blender, then add olive oil, Dijon mustard, apple cider vinegar, salt, pepper, sage and nutmeg and process on high speed until smooth and creamy.
Add the Brussels sprouts to the soup, mix them in and give them a couple of minutes to soften up.
Add the cauliflower cream and stir until well combined. Adjust seasoning, bring back to a simmer and serve.