Heat a large pot, hot hot! Add your sliced onion and cook for a few minutes, use a little of the stock to deglaze as needed. Add the garlic and cook for anther minute or so. Now, add your chopped potato and chopped cauliflower and cook for a few more minutes, stir often. Add the stock and cover allowing it to come to the boil and then simmer for 25 minutes.
Then transfer to a blender and blend well, you may need to add more liquid stock if it's too thick for your liking. Once it's nice and smooth, pour into bowls, sprinkle with spring onions and serve.