Can I bake my au gratin casserole one hour today and finish the last 30 min tomorrow without affecting the flavor?

Certainly. Bake, cool, refrigerate, bring to room temp and finish off.

Some cooks complain that their potatoes turn gray. Easiest solution is to slice potatoes and immediately place covered in water with some acidity such as concentrated lemon juice or white wine vinegar, a tablespoon usually is just enough.

My favorite make ahead scalloped is to slice the potatoes, boil starting with cold water till fork tender. Pull off the stove, drain and add the lemon juice to cool water and immerse the slices for a few minutes. Drain, assemble the cheese and potato layers in the casserole (not more than 3-4 potato layers) and refrigerate. Re heat the next day. Potatoes are guaranteed to be cooked. Try it sometime when the pressure isn't on for all the holiday food to come together at once.