Potatoes au gratin, that decadent and delicious bake-up of cream-laced, cheese-lacquered potatoes, is about as good as simple spuds can get.
To be successful, you need the right kind of potato. Either low-starch or boiling potatoes with a high moisture content and a "waxy" feel will hold their shape when cooked this way. These potatoes are also perfect for soups and stews. Look for local varieties with names like Red Bliss, Ruby, Norland, Red Rose, White Cascade, Kennebec or Purple Vikings.
Or, if they're not marked, choose "boiling potatoes," red potatoes or Yukon Gold and Yellow Finn. On the other hand, high-starch potatoes, such as russets — aka bakers — are not good for gratin, because they will fall apart. They do, however, make perfect fluffy mashed potatoes and, when baked, become puffy and light.
A good potato of any variety is firm, unblemished and heavy for its size. Avoid potatoes with a greenish tinge or those that are beginning to sprout; this means they're on their way to becoming soft and spongy. A potato with green on it is toxic so toss it out.
Store potatoes in a loosely closed paper bag in a cool, dark place and away from onions whose flavor they may absorb. Moisture will cause the potatoes to spoil, and daylight will turn them green. Do not refrigerate potatoes. The thin-skinned low-starch potatoes are not great "keepers," so plan to use them in a couple of weeks.
If you're unsure of the potato type, slice one with a sharp knife. The starchy or baking potato will grab the knife and coat it with a film. If the knife slides through smoothly, then it's a low-starch variety, just right for a gratin.
The French term "gratin" relates to both the shallow dish in which the vegetables are baked as well as the crisp topping of cheeses and, often, but not always, buttered breadcrumbs. So gratin is open to interpretation.
Those favored by Irish cooks, such as author Darina Allen, use stock instead of cream and a range of seasonal vegetables — dark greens, onions, leeks, green onions, watercress, arugula or kale. In the classic French gratin, the rich cream and cheese overwhelm the delicate, nutty potato flavor, but using stock instead allows that distinct earthiness to shine through.
