I used to dabble in [unlicensed] catering for friends and family, once upon a many moons ago, and like all caterers, I learned which dishes can be used to "cheat" when time is of the essence. Gratins are one of those cheats; people love the flavor, it doesn't matter if they're ugly, and they're easy to prepare ahead, leaving the à la minute work for things that can't be done any other way.
In private homes, potatoes au gratin tend to show up at holidays, particularly alongside the Easter ham (unless you're Jewish, in which case it shows up alongside the Hanukkah ham). Often "au gratin" is code for "cooked in milk and salt with two grams of cheese on top"; this doesn't need to be the case.
Sweet potatoes should get some au gratin love, too. This is a great fall side dish and can be made in advance. If you don't feel like pre-roasting the sweet potatoes, just drop the oven temperature to 350°F and bake until it's just done, then jack the heat up at the end (or run under the broiler for a minute or two) to brown the top if necessary.
Tomme de Savoie (available at Trader Joe's) is a nutty, brown French cheese that works great; Reblochon, Emmental (what we know as Swiss cheese), Gruyère or even Brie would work. Just make sure it has some flavor.
1 medium gratin dish (about 9"x9")
4 sweet potatoes (red skin with white flesh work great)
1 cup crème fraîche (substitute thick sour cream)
2 tart apples, cored, peeled and diced
1/2 cup whole milk
1/2 to 3/4 pound cheese (see above), sliced or shredded
Salt and pepper
Butter and oil
1. Rub each sweet potato with oil and prick with a fork. Lay directly on the rack of the oven, set the heat to 375°F and leave for one hour. Remove and let cool (at least enough to handle).
2. Preheat the oven to 400°F.
3. Peel the sweet potatoes and slice into thick (at least 1/2-inch) rounds.
4. Butter the gratin dish liberally.
5. Put a single layer of sweet potatoes in the dish; smear half the crème fraîche on them and scatter half the apples on top. Salt and pepper the layer lightly.
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6. Repeat with a second layer of sweet potatoes, crème fraîche and apples; salt and pepper.
7. Pour the milk evenly over the dish.
8. Top with the cheese and bake, uncovered, for 30 minutes until the cheese is bubbling and browned in spots.
9. Let cool slightly before serving; not to room temperature, but not boiling hot either.