Traditionally shepherd's pie is made with lamb. However, over the years ground beef has become a frequent meat substitute for lamb in this recipe. Since our family eats ground beef more, this is why we used it. But, if you want to stick to traditional recipes, you can certainly use ground lamb. You may find recipes referring to shepherd's pie with beef as Cottage Pie. So, technically this is a Cottage Pie recipe. But most people (at least in the US), refer to this as a Shepherd's Pie whether it uses lamb or beef. And you can substitute ground turkey or the even ground pork in this homemade meal as well.