Allow mashed potatoes to fully cool in the fridge.
Freeze mashed potatoes in different portions to give yourself options. Use larger pans for family-sized servings or freeze in muffin tins or individual portions.
Reheating - Oven Method
Cook the potatoes uncovered at 350F for 20-30 minutes, stirring occasionally. About halfway through the heating, I like to add a few slivers of butter to the top of the spuds.
Reheating - Slow Cooker Method
(This works best if the mashed potatoes have thawed before reheating).Transfer the potatoes to the slow cooker and cook on low for 2-4 hours, stirring occasionally.
Reheating - Microwave Method
Reheat in a microwave-safe dish, cooking for 5 minutes at a time. Stir in-between cooking, and add additional butter/cream if needed.
Reheating - Stovetop Method
(This works best if the mashed potatoes have thawed before reheating).Cook in a heavy-bottomed saucepan (like enameled cast iron) on low, stirring occasionally until warmed through (~20 minutes).
When you’re making the mashed potatoes, try not to add excessive liquid. It is ok if they seem a bit “dry” before freezing. There will be some moisture released when they thaw/bake and you can always add additional milk/cream/butter/stock while they are reheating.