This was another staple in my house growing up in Ireland. Because we have such incredible lamb in Ireland, we cooked with it a lot. There are so many variations of this classic traditional dish, I like to keep mine simple but adding a colcannon mash on top. You could substitute the leeks with cabbage, one tablespoon Dijon mustard, or three cloves of crushed garlic. If you make it with beef instead of lamb then it becomes a cottage pie. You can cook the filling ahead and freeze it if you wish and then add the potato topping after you have defrosted it. If I have leftover lamb from a roast, I will use it in this pie.