Heat 1tablespoon of the oil in a large saucepan over medium-high heat. Add the prosciutto and sauté until crisp. Drain half the prosciutto on paper towel, leaving the rest in the saucepan.
Reduce the heat to medium. Add the remaining oil and the onion to the saucepan and fry for 5 minutes, or until softened. Add the garlic and cauliflower and fry for 3 minutes.
Add the beans and 1litre (35fl oz/4cups) water and season well with salt and pepper. Bring to the boil, then reduce the heat and simmer, covered, for 15 minutes, until the cauliflower is tender. Set aside to cool for 10 minutes.
Purée the soup until smooth. Season with salt and plenty of pepper. Stir in the cream and gently reheat the soup. Serve immediately, with the crisp prosciutto and chives sprinkled on top.