Fry the prosciutto in a dry saucepan over a high heat until crisp, then remove half with a slotted spoon and set aside. Turn the heat down to low, pour in the oil and tip in the fennel with a pinch of salt. Cook for 5 mins until softened, then throw in the garlic, chilli flakes and thyme and cook for a further 2 mins, then pour in the wine or stock and bring to a simmer.
Tip both cans of butter beans into the stew, along with their liquid, then add the tomatoes, season well and bring everything to a simmer. Cook, undisturbed, for 5 mins, then stir through the kale. Once wilted, ladle the stew into bowls, removing the thyme sprigs and topping each portion with the remaining prosciutto.