This is the classic shepherd's pie recipe you've been searching for. Browsing the ingredients list, you may have noticed that Shepherd's pie consists of quite a few ingredients, but it's actually just leftover food reimagined. In fact, if you look at many of the ingredients called for in this recipe - mashed potatoes, gravy, peas, roast beef - you will recognize many Sunday roast dinner staples. So, essentially, this decadent casserole is a budget-friendly food that will help you reuse leftovers.