1. Cut the cauliflower florets from the stem, cut 1 cup of the florets into small pieces and set aside.
2. Melt 4 tbsp. of olive oil in a large saucepan over medium heat, add the onions and a pinch of salt. Fry until onion is softened, but not browned.
3. Add half of the large pieces of cauliflower and all of the sweet potato to the pan, add the stock and bring to the boil.
4. Reduce the heat to a gentle simmer, and cook for 15 minutes.
5. Add the remaining large chunks of cauliflower to the large saucepan, and cook for a further 15 minutes until all the cauliflower and sweet potato are tender.
6. In another small saucepan, melt the butter and 2 tbsp. of olive oil. Add the cup of small florets and cook on medium heat until they are browned. Stir occasionally, and remove florets once they have good colour – but not black! Remove the florets with a slotted spoon, and place on paper towel to drain excess oil. Sprinkle generously with sea salt.
7. Blend the soup with a stick blender or place in a blender until smooth. Return the soup to the saucepan, and then add the vinegar and salt and pepper (watch the vinegar being added, it adds vibrant colour as it reacts with the soup!)
8. To Serve, drizzle with pouring cream and sprinkle chopped chives and the crispy florets on top.