I'm serving them for our thanksgiving meal tonight but was hoping to ready them now.
A couple of hrs ahead works for me; I don't prepare several hrs. ahead because I do not want all of the cream/milk liquid to be fully absorbed before baking. One of my favorite side dishes to serve; use a good quality grueyre; makes all the difference.
In my experience, this this dish tastes best when first cooked. Though it reheats just fine, it's better freshly baked. It's also super easy to make, so prep your cheese and wash scrub the potatoes any old time, but save assembly for last possible minute. Unfortunately, I've never found a way to hold peeled or sliced spuds to hold for more than a few minutes. You need an assistant. I'd enlist a sous chef, possibly with a small bribe like an early cocktail or a taste of some 30-year-old single malt.
And if the sous is very short (=under 18), let the bribe be dessert - as in ever-needed quality control tasting of pie, cake etc before being served.
My answer was to the original question "to prep the dish ahead; NOT "to bake the dish ahead". I would never "bake" the dish ahead. Perhaps the second responder did not address the original question???
I think my wording may have been unclear; I wouldn't bake it ahead but was thinking about how far I could go with the prep. Currently potatoes are peeled and submerged and cheese is grated. My instinct was to leave it there so I got just the confirmation I needed, thank you!
My recipe to prepare them to bake ahead is from a NY caterer and has worked for me for years; but of course you do whatever works best for you. For others who want another option, I do not like to leave the potatoes in water because of its effect, so I prepare the entire dish up to 2 hrs ahead; cover; and baked for the dinner party in enough time for them to rest before serving.