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Sweet potato gratin is a baked dish in which sweet potatoes are combined with cream and savory spices in a casserole dish. Most of the time, the gratin is topped with bread crumbs or various grated cheeses. It is generally served as an accompaniment to meat preparations or other substantive entrees.
The “gratin” form of cooking originated in France, but sweet potato preparations are most popular in North and Central America, where the sweet potato is both native and widely cultivated for consumption. Different cooks have different ways of putting sweet potato gratin dishes together, but most final products share three characteristics. All are baked, for starters. They usually involve thin slices or small chunks of raw sweet potato. Finally, all are generally savory, using sharp cheeses, fresh herbs, and potent spices to balance the potato’s natural sweetness.
Most gratins are assembled in layers. Cooks will arrange thin slices of sweet potato in a shallow baking dish, then add a splash of cream or whole milk. Other ingredients like caramelized onions, roasted vegetables, or even other varieties of potato can also be added.
Meat is one of the only things that is almost never incorporated. Adding meats would make a sweet potato casserole that could serve as its own main dish. Gratins are almost always designed to be side dishes or accompaniments, and are usually best kept simple.
Cream is almost always an important component when making au gratin dishes, particularly when sweet potatoes are involved. Uncooked sweet potato is usually dry and quite dense. Baking the potatoes in liquid is one way to tenderize them, as they absorb moisture when heated. Choosing rich cream improves the sweet potatoes’ flavor, and also adds a certain decadence to the dish as a whole.
More diet-conscious cooks can use milk or thinner liquids like chicken stock in sweet potato gratin, though these will impact the overall consistency of the dish. One of the most characteristic traits of a sweet potato gratin is its consistency. What goes into the oven as a dish full of separate components comes out as a unified whole that holds together, even when sliced. Cream in many respects acts as a binding agent, causing the potatoes to stick to each other. Other liquids will moisturize the potatoes, but the presentation often suffers.
While it is possible to create a sweet or sugary au gratin, this is uncommon. Some cooks will add cinnamon or other sweet spices to a sweet potato gratin, but these are usually balanced with pepper, cheese, and herbs. Many sweet potato preparations are designed to be topped with crusts of sugar and candied nuts, but the gratin is not in this category. Gratins are generally seen as a means of making sweet potatoes savory.