Store leftovers in an airtight container in the refrigerator for up to 3 days. For longer storage, freeze the mashed potatoes and beef gravy in separate containers for up to 2 months. To reheat, gently warm mashed potatoes in the microwave with a splash of milk to loosen them. Reheat the beef and gravy on the stovetop over medium-low heat, stirring occasionally and adding a bit of broth or water if needed.