Unlike soups, stews, braises, and chillis, mashed potatoes do not taste better after an overnight rest in the fridge. Cold potatoes are stiff and dry — mere shadows of their former selves. But nuking them in the microwave or stirring them ferociously over a hot burner can only make them dryer and — if you agitate them too much — gluier. For truly luxurious leftover mashers, you have to heat them gently and with love (more fat).
I actually have had some success microwaving cold mashed potatoes, but I do it in small 15-20 seconds bursts, gently folding in half & half, milk, and/or chunks of butter between each blast. For these purposes, a 180-degree Celsius oven will work just as well. Whatever the heating method, adding extra moisture and fat is non-negotiable, as they are what will revert your stiff, dry potatoes into luscious, creamy potatoes. For the moisture, you can use cream, milk, or even stock, but be sure there’s some fat in there as well, whether butter or a vegan alternative (if that’s your bag).
The more you work the potatoes, the thicker and stickier they become, but a bit of gentle folding won’t do to much damage, especially if the spuds weren’t overworked to begin with. If you’re reheating a small amount of potatoes in the microwave, place them in a bowl with a few pats of butter (I use around a tablespoon per serving) and add a splash of cream, half & half, milk, or broth. Heat for 20 seconds, gently stir with a fork, add another splash of cream, and heat again. Repeat until your potatoes are hot and creamy.
If you’re reheating a larger portion in the oven, you can be a little more hands-off. Spread the potatoes out evenly in an oven-safe tray, dot it with butter (about half a tablespoon per cup), and pour 1/4 – 1/2 a cup of cream, half & half, milk, or broth (or a mixture of broth and dairy) on top — you want the potatoes to be barely covered.
Tightly cover with foil and warm in a 180-degree Celsius oven for about half an hour, until the potatoes reach a temperature of 75 degrees Celsius. Fluff with a fork and serve.