Can I make a potato leek gratin a day ahead and refrigerate before cooking?

I would bake as directed. cool down, wrap tightly and refrigerate. Gently re-heat when your ready.

I think you are right to be concerned about the grey. is this a 52 recipe or a linkable recipe (because you can send an email to a 52 recipe chef via their member pg.) I would do exactly as you thought and if I were doing it (dairy or no dairy) i would leave it out at room temp overnight- in oven - til i finished baking it the next day. (But that's me; in the winter i rarely get all hot and bothered about leaving out non-meat cooked foods.) If you feel you must refrig it, make sure to take it out of frig a good 3-4 hours before baking it(it's winter here in my house). But obviously, if you're in the tropics, that suggestion is irrelevant!