Heat the olive oil in a saucepan and then gently fry the pancetta until it is lightly golden. Add the garlic and leeks and fry for a further 5 minutes until the leeks are softened.
Stir in the drained butter beans and bay leaf. Pour in the stock and bring to a simmer. Using a potato masher, roughly mash the butter beans, keeping some whole but creating a thicker broth consistency.
Add the broccolini and Dijon mustard, and cook for 5 minutes, until the broccolini is tender. Season with salt and pepper and serve hot.