Cut the cauliflower into small florets – you should end up with around 450g after trimming. Put into a clean saucepan with the butter and cook it gently for about 5 minutes. Pour over the chicken stock. Cover the pan and bring to a simmer, then cook for around 12–15 minutes, or until very tender – don’t be tempted to add extra water or the finished soup will be too thin.
Remove the leaves from the rosemary sprig and put in a pile on a board with the sea salt. Chop the rosemary and sea salt together until very fine – a coarse chopping will not do. Set aside.
Add half the beans to the soup and blitz with a stick blender until smooth. Stir in the rest of the beans and the double cream. Loosen with a little milk if it’s too thick. Warm through gently, then season to taste with a touch of sea salt and some black pepper – show modesty with the salt, as more will be added when scattered about with the rosemary.
Ladle into warmed soup bowls. Sprinkle with the rosemary salt mix, a grind of pepper and a good grating of Parmesan, and scribble over with a little olive oil to finish.