Heat the oil in a large casserole. Tip in the lamb and cook for 5 mins until any liquid has disappeared, then add the onion, carrots and leeks. Cook for 5 mins more, stirring often, until the veg is starting to soften.
Pour over the stock, add the rosemary, cover with a lid and cook over a low heat for 1 hr. Stir in the beans and cook for 30 mins more, topping up with water if necessary, until the lamb is tender and cooked through. Serve with some crusty bread or potatoes, if you like.