The gratin can be made early in the day; reheat in a 325° oven.
Gratins are wonderfully simple side dishes that welcome experimentation. For example, play around with the infusion in Step 1. Instead of (or in addition to) thyme, try rosemary or sage or add the white part of a roughly chopped leek to the mix.
In addition to the potatoes here, you could add some peeled, thinly sliced turnips or sweet potatoes for some great color. For a garnish, try some snipped chives.