trying to make ahead cauliflower gratin. should I let ebrytbing cool down then assemble or what?

I personally prefer second day old gratin. I would bake it, let it cool and then refrigerate. Then if serving in the next few days, I would put it out for 30 to 60 minutes and then warm in oven at maybe 300 degrees/45
or so minutes. Use a toothpick to insert in middle and then touch to see if it is warm. You may want to place aluminium foil on top so it does not dry out.