I'd like to prepare smaller portions and freeze the Make-Ahead Mash, without the topping. I understand that they should only be reheated once after the initial preparation.
NIgella's Make-Ahead Mash is a mashed potato gratin dish that can be made ahead and then reheated in the oven. It is particularly useful for entertaining large crowds. Freezing mashed potatoes is slightly tricky as they can become watery when thawed and don't always emulsify again on reheating.
If you did want to try freezing the potatoes then we would suggest freezing them in individual portions (approximately 200g/¾ cup per portion) in freezer and oven proof dishes. Wrap the dishes tightly and freeze for up to 1 month. Reheat the potatoes direct from frozen in an oven preheated to 180°C/160°C fan/350°F - the potatoes should take around 30 minutes to reheat but check that they are piping hot all of the way through before removing them from the oven. You can freeze the crumb topping in a separate container and sprinkle a little, direct from the freezer, over each dish before baking. Please note that we have not tried this method, so are unable to guarantee the results.