Heat the oil in a pan. Add the white part of the scallions (reserving the rest), and the garlic and sizzle for 2 minutes, then add the cumin and cook for a minute more.
Tip in the beans and a splash of water then, using a potato masher, break the beans down as they warm through, to make a rough purée. Add another splash of water and stir to a creamy consistency. Continue to cook until piping hot.