Melt the butter in a medium saucepan, add the leeks and cook for 3–4 minutes until softened. Stir in the butter beans, cream cheese and milk and cook for 2 minutes until the butter beans are heated through and the sauce is bubbling. Season to taste.
Meanwhile, mix together the breadcrumbs and cheese.
Transfer the bean mixture to a warmed ovenproof dish, sprinkle with the breadcrumbs and cheese and cook under a preheated hot grill for 2 minutes or until the topping is golden. Serve immediately.