Doing them a la carte I cook them to order and then process them in a paco jet with s&p butter and a bit of creame fraish. Honestly, in Almost 40 years of cooking professionally I haven't come up with a way to reheat mash and have it be good.
For me mashed potatoes are one of the most miraculous dishes. About as inexpensive as can be and easy to make but taste fantastic. Everyone loves mashed potatoes. As a restaurant owner, you can't ask for much more from a dish. Boil potatoes, add milk, butter salt and pepper.
Please don't be one of those cooks who can't or won't do them fresh. I have lost track of the humber of times I have ordered a meal with mashed potatoes only to discover they were from a box or simply treated carelessly and callously. Is there any other dish that delivers so much for so little effort? Whatever it takes, make them fresh, every time.
I have to say, I get pretty good mileage out of my mashed spuds, i.e. I successfully reheat them once, adding some more milk in the process. Admittedly, it only works with one particular potato variety, Fontane, which I use for both fries and mash over here in Germany.
Yes you can reheat mash with no problems at all. Just cook your potatoes as you would normally do, add your milk, cream, butter and s+p. Just add a little bit more then what you usually do. Then store in fridge, put them in portions if you like. IF you just gonna reheat one Serv, just use microwave! And if it's a bigger size, use a steam oven!