Place the beans, garlic and stock in a saucepan, bring to the boil and simmer for 10 minutes. Remove from the heat, drain off any excess liquid, then mash with a potato masher and stir in the parsley. Keep warm.
Heat half the oil in a nonstick frying pan, add the leeks and fry for 2 minutes, then add the measurement water. Cover and simmer for 5–6 minutes until tender.
Meanwhile, heat the remaining oil in a small frying pan, add the scallops and fry for 1 minute on each side. Serve with the white bean purée and leeks.