Can you prepare turnip au gratin the day before?

You can, but I don't recommend it. To prevent the potatoes from oxidizing and turning dark, you must prepare andpre-bake before storing in the refrigerator. The potatoes, however, tend to soak up the sauce while they chill and so the dish also tends to be a bit dry when you reheat it. There are many processed food manufacturers that offer frozen au gratin potatoes so I'm assuming this is an option, but I have never tried it at home.

There are recipes available that allow you to bake this dish in the microwave. It does shorten the overall cooking time, but in my experience the potatoes are often very rubbery in texture when cooked this way. The recipes also generally require you to cook in short increments and stir often in between. I find the frequent interruptions when I'm putting the rest of the meal together very disrupting, but if this doesn't bother you and the chewy texture is acceptable, a microwave recipe may be viable alternative method for you rather than preparing ahead of time.

For me the best way is the old-fashioned one: prepare and stick in the oven for the 60 - 90 minute baking time it requires. At least this frees you up to complete the rest of the meal while the dish bakes. It's an easy dish to pair with proteins that you can cook on the stovetop: pan-seared shrimp or scallops over a salad, or veal or chicken breast cutlets with sauteed vegetables, for example.