You can also freeze unbaked Shepherd's pie. I make the pie in Pyrex containers (glass ovenware, that allows you to move from freezer to oven and oven to freezer safely). Make sure the pie is completely cold, then cover it well with cling film or with an airtight lid and freeze.  Freeze the pie on the day that you make it. Defrost in the fridge overnight, then cook as per the recipe. Alternatively, you can cook it from frozen. Heat the oven to 180C/360F cover the pie with foil, and cook for 1½ hrs.