POTATO GRATIN

A POTATO GRATIN—ALSO KNOWN AS SCALLOPED potatoes—consists of thinly sliced potatoes covered by a liquid and baked in a wide, shallow baking pan until the potatoes are tender and the top gets a burnished crust. Most recipes follow the same procedure: Rub an ovenproof dish with garlic, brush it with butter, layer it with sliced potatoes, add seasonings and just enough liquid to cover the potatoes, and bake.

We tried all the major potato varieties, including all-purpose whites, boilers, yellow-fleshed, and new potatoes. All made tasty gratins, but we did find subtle variations in flavor and texture—waxy potatoes ended up firmer, for example, while floury baking potatoes were more tender. However, the differences were relatively slight, and we began to suspect that the way the potato is sliced is more significant than which type is used.

We found that potatoes cut into ¹/₁₆-inch slices soaked up the cooking liquid and melted into a cakelike texture. When sliced ¹/₈-inch thick, however, they kept their shape nicely throughout baking but still melded together. When sliced too thick (¹/₄ inch or more), however, the potato slices did not properly meld together in the oven. Don't slice potatoes in advance and hold them in water. We found that this caused their starch to leech out, resulting in a bland gratin with layers that do not hold together properly.

The pan in which you make a gratin is an important consideration. A standard gratin dish is oval, 10 to 12 inches long, and 6 to 8 inches wide. As it turns out, the dimensions of the pan are more important than the material it's made of. We cooked successful gratins in a variety of dishes, from classic enamel to earthenware to ovenproof glass—the key is that they were all shallow. Two to three layers of potatoes should come about three quarters of the way up the sides of the pan so they cook evenly and have a broad surface area to brown.

We found 350 degrees to be the best oven temperature. At higher temperatures, the slices on the top of the gratin tended to dry out and become tough. We also discovered that it helps to baste the top slices once or twice during the cooking so they stay moist and brown evenly.

We made gratins using a variety of liquids, including whole milk, low-fat milk, half-and-half, heavy cream, meat broths, and water. The heavy cream had a tendency to break up and become greasy, particularly at higher temperatures. The low-fat milk, on the other hand, was insipid, while the whole milk was almost completely soaked up by the potatoes, leaving only a light curd between the layers. Our favorite all-purpose dairy cooking medium turned out to be half-and-half. The gratins we made with it had just the right balance of saturated potato and saucy liquid, without overwhelming the taste of the potato. They were also easy to prepare.

Until this point, we had tried pouring cold liquids over the layered potatoes and baking them. We wondered if we could cut the cooking time by heating the potatoes and liquid beforehand. We placed a pound of sliced potatoes, enough liquid to barely cover them, salt, and pepper into a saucepan and brought the liquid to a boil, stirring occasionally so the potatoes didn't scorch or stick. We next lowered the heat and simmered the mixture until the liquid thickened. Finally, we poured the potatoes into a baking dish and finished them in a 350-degree oven.

This method proved to possess many benefits. The cooking time was reduced, the assembly of the gratin took less time, and, because the seasoning was added to the liquid, the final product was evenly seasoned.

There are several ways to enhance the crust from which these dishes get their name (the French word gratin means "crust"). Dotting the top with butter makes for a golden crust, as does cheese. A nice finish can also be achieved by pouring a thin layer of cream over the top of the gratin for the final 20 or 30 minutes of baking. This cap of cream browns evenly without greasy edges.