That’s a very good question. It is probably how I would do it for a pot luck or large crowd too. This is possible and I will outline a few tricks you’ll need to know. To cook the potatoes, slice them thin for the casserole and add to a pan with cold water, bring to a boil and then simmer the potatoes. They will be fully cooked when you can easily pierce them with a sharp knife or mush with the tines of a fork. I usually pull out a slice when I think they are done (typically 15-20 minutes) and let it cool enough to taste. Once the potatoes are cooked, add some concentrated lemon juice to the water, as the little bit of acidity will help the potatoes from turning brown from oxidation. Let sit until room temperature and then drain and place in a glass pan (not metal) and refrigerate before using the next day. This is the perfect way to make sure the potatoes layered in a casserole dish are cooked when heated and serving.

The only downside… By slicing and placing in water a lot of the natural potato starches will leach out when cooked and drained off. Be sure to adjust your recipe so that the milk or cream is slightly reduced. I recommend using whole milk or cream, not low-fat milk. If adding grated or sliced cheese, add a little more for extra binding. Tossing the cooked potatoes with a whipped or beaten egg also helps bind the layers when baked off, but don’t do this if planning to transport to the event.

There are a lot of recipes and preparation suggestions at the following link:

https://idahopotato.com/foodservice/potato-101-au-gratin-scalloped