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I have a problem with potato gratins. It's the same problem I have with muffins—I only want the top and usually neglect what's below deck. The interior of a poorly made potato gratin is bland, dry, and crumbly, but even the worst gratins have a savory and speckled crust. I'm not alone with my prejudice. Jeffrey Steingarten, the author of The Man Who Ate Everything, developed an indulgent single-layer potato gratin recipe that yields a crusty-top-only dish. I've made his recipe several times and can admit to curling up on the couch with it, picking at every golden bit with my fingers. Although I delight in this shameful activity, some nights I'm struck with guilt, worrying about that under part of a gratin that's so often ignored. This is what motivated me to perfect the classic potato gratin—one with a flavorful and creamy interior not to be dismissed, and a bottom that rivals the top's golden hue.
Like many classic dishes, recipes for potato gratin are all over the map. The basic idea is always the same—sliced potatoes are layered, smothered in cream, milk, or a sauce, and baked together until tender and golden brown. Some recipes insist on using cream only while others stand by milk. There are vehement anti-cheese believers with equally zealous cheese proponents across the aisle. Then there are the extremists like Kenji, who turn the whole dish on its head with his sideways-loaded hasselback potato gratin.
I just want the classic. For me, this means a gratin that's crisp all the way around—with cheese and potato that has browned on the bottom, sides, and top. The potatoes are tender and well seasoned, with layers of creamy curds in between them. Most importantly, I want to want to eat it all, and not just think about picking off the top layer when no one is looking.
To figure out what I was up against, I tested all the variations: different proportions of milk and cream, liquid-to-potato ratios, baking times and temperatures, and even went old-school with a few batches with whole eggs and yolks.
Milk vs. Cream
As a gratin bakes, there are two actions that are producing the creamy filling. Firstly, the potatoes release some of their starches, which thicken the surrounding liquid. Secondly, the dairy reduces—some of its liquid is absorbed by the potato while the rest slowly evaporates. As the liquid in the dairy is reduced, protein and fat is left behind, forming creamy curds throughout.
In the testing, I found that too much cream breaks during the cooking, coating the potato slices in a slick of fat and masking their earthy flavor. On the other hand, gratins made completely with milk were dry and filled with crumbly curds. The combination of the two produced the best result. I found that a higher ratio of milk than cream added just enough fat to produce extra silky curds without becoming too rich. I also tested a batch with evaporated milk, fully expecting an even better result due to its inherently high concentration of protein, only to find no difference in the final dish. Since I prefer to use fresh ingredients, I decided to stick with regular milk instead.
Some old cookbooks recommend the addition of a whole egg or yolk into the dairy mixture. Even the smallest amount of egg resulted in the gratin setting up with the brittle texture of a flan. It sliced well, but I preferred the soft and supple texture of the gratin without egg over picture-perfect serving portions. If the latter is what you're after, the addition of half a yolk to the milk and cream mixture is enough to achieve a clean slice.
The next challenge was to introduce layers of flavor to a dish that could easily fall flat. To do this, I steeped the milk and cream with garlic and shallots for their pungency, black peppercorns for a mild heat, and thyme for it's fresh, piney aroma. After an hour, the milk mixture was thoroughly perfumed and the aromatics were ready to be strained out. This infuses the gratin with layers of complex aromatics, without interrupting that silky-smooth texture.
I then seasoned the milk and cream mixture with a touch of freshly ground nutmeg and a generous dose of salt. You have to over-salt the milk mixture at this point so that it can properly season the potatoes. I also found that the gratin was more consistently seasoned when the salt was dissolved in the milk mixture rather than sprinkled in between the slices of potato.
Consider the Potato
Because I want a gratin filled with creamy and tender potatoes that are also crisp on the sides and top, I turn to high-starch russets. These potatoes are low in moisture and therefore best suited to absorb the flavorful milk and cream mixture, quickly becoming plump and tender. Their starch content is also important because as they cook they release plenty of starch molecules, which will help thicken up the gratin's liquid component.
On the downside, high-starch potatoes also oxidize faster, quickly becoming brown and discolored when they are cut or sliced. To remedy this, many recipes say to hold the sliced potatoes in water to halt the oxidation until you are ready to use them. The problem is that this washes off some starches and results in the loss of vital thickening power. Instead, I slice them directly into the milk they'll be baked in, preventing the potato slices from oxidizing while keeping all their starches right where you want them—in the final dish.
The Cheese Question
Some recipes insist that there's no place for cheese in a potato gratin—the cheesy flavor and creamy texture should entirely come from the milk proteins concentrating and browning. However, I like the brightness cheese brings to the dish. Too much cheese will break and become greasy, but a small amount of Parmesan adds a salty tang, while some Gruyere puts nuttiness into play. More importantly, the cheese forms a crunchy-chewy crust along the bottom and sides of the dish, wrapping the gratin in a savory frico—a crispy cheese cracker.
Putting it All Together
After rounds of testing, this perfect classic potato gratin comes together in just a few easy steps.
Step 1: Make a Flavorful, Protein-Rich Base
The dairy base needs to have a higher proportion of milk to cream. As it reduces, there will be more concentrated proteins than with cream alone. These proteins form curds that brown, adding a custardy texture and rich flavors. Also, starting with more total dairy in your gratin will yield lots of rich, brown curds. It will look like far too much milk, but have faith that it will eventually all cook down.
I combine the dairy with aromatics and bring to a boil, before covering to steep. Taking your time with this step is the best way to ensure a flavorful and seasoned final dish. After steeping, I bring the milk mixture up to a boil before straining into a bowl. Reheating the milk mixture cuts back on your total bake time and gets the potato slices soaking up all that flavor immediately. The recipe still works without this step, however it may take up to half an hour longer to bake in the oven.
Step 2: Maximize Starchiness
I start with high-starch russets and hold on to all that starch by slicing peeled potatoes directly into the hot infused milk. The thin, rigid slices immediately wilt, releasing their starches to be distributed through the milk-cream mixture. I also give them a good stir so all the slices are fully coated in milk and they begin to absorb the seasoning and aromatics.
Step 3: Get a Little Cheesy
I like a little cheese for extra flavor and to bump up the browning. After the potato slices are shingled in a buttered casserole dish, I sprinkle a touch of grated parmesan and Gruyere before topping with the next layer. I'll repeat this until no more potatoes remain, leaving the top layer cheese free for the initial low temperature bake. The reserved starchy milk gets poured over the layers.
Step 4: Two-Temperature Baking
I first bake the gratin uncovered at 300°F for about one and a half hours. This low temperature gently reduces the milk and cream, while the potatoes become tender. At a higher temperature the milk mixture might bubble up over the sides of the dish, leaving your gratin high and dry.
After the first low bake, I top the gratin with a little more cheese before blasting it under the broiler. The sides and bottom will be richly golden and that extra dusting of cheese will form a thick and chewy crust.
By starting with a flavorful base and focusing on a few key details, no more gratins need to suffer the fate of having an unappealing lower half. If only someone could convince me to eat a muffin stump, my conscience could finally rest at ease.