as long as the potatoes are submerged in the creamy cooking liquid, you will not have an issue of it turning brown/grey. there is no need to par-bake or parboil the potatoes. to ensure that they will not turn brown/grey, but there is not enough cooking liquid to necessarily cover, cover the dish with plastic wrap that is touching the top of the potatoes and then place a lid or something on top of the plastic to push them down into the liquid. of course you could avoid it turning brown/grey at all by cooking it the day before and just warming it up for service.
Definitely not the night before. I do, however, do mine in the morning and then cover it with plastic wrap directly on the surface of the potatoes. I've never had any issues when I've done it this way
I've had mixed luck with parbaking. The pieces of potato that were not totally submerged because greyish-purple overnight. I may not have cooked it long enough. I'd probably cook it, covered tightly with aluminum foil, for nearly the full length of time recommended, and not put cheese on top at that time (if I were doing so at all). Then I'd bring to room temperature before heating in the oven uncovered. Others may have more helpful insights. ;o)
This came up on top chef this season. You can put together the whole gratin and par bake it - half or three quarters of the time. Then bake it to completion before dinner the next day. It will probably be even better with the melded flavors.
They will turn an odd color for sure as they begin to ferment. I suppose you could parboil the potatoes, but that just adds another layer of work. Best to put a gratin together just before putting it in the oven.
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