If I make my mashed potatoes today, (they contain Boursin cheese), should I refrigerate overnight to be slowly reheated tomorrow with some wa

Yes you can refrigerate them overnight. I agree with Sarah that tomorrow, you may need to add a little bit of *warmed* milk to loosen them up again. You can warm them up, covered with a dishtowel, in a bowl set over a pot of very gently simmering water, giving them a stir every now and then. Or in a crockpot. They sound delicious!

I would refrigerate, then slowly reheat them on the stove with some warm milk!

I would refrigerate, then slowly reheat them on the stove with some warm milk!