The Food Lab: Hasselback Potato Gratin (These Might Be the Best Potatoes Ever)

The Food Lab

Unraveling the mysteries of home cooking through science.

[Photographs: J. Kenji López-Alt, unless otherwise noted. Video: Natalie Holt]

Every online food recipe trend has its watershed moment, the point at which it goes from being a fun project to going full-on viral. It's usually when bacon gets added to it. It's the moment when every blog and Instagram feed is so saturated with it that even mainstream media will pick it up. For Hasselback potatoes, that moment was in early 2011.

You've probably seen them before. According to some unsourced online references, the dish was created at the Hasselback restaurant in Stockholm sometime in the 18th century (this was way before the internet started bacon-ing everything).

Grilling: Hasselback Potatoes with Garlic and Parmesan

[Photograph: Joshua Bousel]

At their simplest, they're made by slicing potatoes at regular intervals almost all the way through, then separating the ridges, adding a bit of butter or oil in between the ridges, and baking them until they're crisp and creamy. They come out looking like, well, like nothing else I can really think of. Dragon's eggs? Bad guys in a Super Mario game? Point is, they look awesome.

But, to be absolutely honest, I've never been too happy with the way they taste. They're supposed to combine the best part of a baked potato—the creamy, moist interior—with the best part of roasted potato chunks—the crispy edges—but really, they produce only mediocre versions of both. The interior is never as moist as I want it, and the edges are always more dried and leathery than really crisp.

The main problem is that a Hasselback potato loses lots of moisture as it bakes because of all the slits. The interior ends up drying out unnecessarily. At the same time, the ridges on top don't crisp as well as they should because, in order to get truly crisp edges, like in a French fry or a good roast potato, you need to first gelatinize the starch inside the potato to form a continuous structure. This requires moisture.

I spent a few dozen batches' worth of potatoes trying to fix these deficits before I woke up in the middle of a Tuesday night, turned around, shook my wife awake and said, "Adri, Adri, you must awake! I just had an idea, and I must peel some potatoes. Make haste!"

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She gave me the usual You interrupted my favorite activity—sleeping—to ask me to do some menial kitchen work? How about this idea: You go do it yourself, then make yourself comfortable on the couch look before rolling back over and nodding off. Sometimes I just don't understand her.

Nevertheless, I went into the kitchen and got to work on the first batch of what would end up being my favorite potato recipe in years.

Here's the idea: What if I were to take the creamy interior and the crisp edges to the extreme, combining the concept of a Hasselback potato—that array of crisp ridges at the top—with a creamy potato gratin, the king of all casseroles? What if, indeed.

It's a sideways potato gratin, if you will.

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The dish starts out just like most potato gratins: with sliced potatoes. The concept of a Hasselback potato, with the ridges all cut into a single intact potato, is neat, but it's not particularly practical. Nor, as it turns out, is it the optimal way to get the most crisp edges in your dish. What we really want is to stack together many slices of potato that vary in size, so as to create plenty of bits that stick in and out. More surface area for better crisping. As it turns out, this also greatly reduces the amount of fiddly prep work required.

If you've got an inexpensive mandoline slicer, then it's a snap. I tried it with both peeled and unpeeled potatoes and preferred the cleaner crunch you get from peeled potatoes.

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From there, it progresses like a standard gratin: I mix heavy cream, grated cheese (I used Comté and Parmesan), picked thyme leaves, salt, and pepper in a bowl. (About a cup of cream per pound of potatoes works.)

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Then I add the potatoes and toss them all together. This step is worth taking your time with: It's crucial that every single potato slice gets coated on all sides with the mixture. That means prying or sliding apart all the slices of potato that are stuck together and dipping them into the fatty mixture.

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That cream not only adds flavor and moisture, but also keeps the potato slices from sticking together as they cook. Without it, starches released when the potatoes are sliced swell up, sucking up extra moisture and turning the interior of the casserole gluey and starchy instead of creamy and tender.

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Here's where we turn things on their head side: Rather than stacking the sliced potatoes horizontally, as in a traditional gratin, I pack them into a greased baking dish standing on their edges, working my way around the perimeter of the dish and packing it all in tightly. Because potatoes vary wildly in exact shape, you end up with tons of little nubby bits sticking out all over the top surface.

Nubby bits that will hopefully crisp up as they cook.

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To get the interior extra creamy and moist, I add all the excess cream/cheese mixture to the casserole. Your potatoes should end up about half submerged, with a few stray pieces of cheese and thyme strewn across the top.

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So what's the trick to getting the top crisp instead of leathery? Well, the coating of fat helps, for starters. It prevents excess evaporation of moisture, which allows the starchy potato slices to partially gelatinize before they dry out fully. But I found that, for optimum crisp-tender texture, I needed to adopt a two-stage cooking process, similar to how I cook my Ultra-Crispy Roast Potatoes.

I start out baking the casserole covered in foil, so that trapped moisture steams the potatoes until they're tender, followed by a good cook uncovered to dry them and brown them.

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As the potatoes cook, the cream eventually starts to boil, simmering up and over the tops of the potatoes, basting them as they roast, aiding further in preventing them from getting leathery.

Another layer of cheese, added halfway through the uncovered portion of the cook, adds a layer of flavor to the casserole.

During the last stages of cooking, the cream eventually loses enough moisture that it breaks, releasing its butterfat, which coats and then gets slowly absorbed into the potatoes as they continue to lose water content. Milk proteins in the cream and the cheese coagulate, creating little pockets of curd-like tenderness between slices.

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The final dish is nothing short of glorious. Look at it. I mean, look at it. Every bite has a combination of crisp but moist upper potato ridges and rich and creamy potatoes underneath, with cheese underscoring the whole affair.*

* I strongly believe that more affairs ought to be underscored with cheese.

It's so good that I've decided to make this only in the middle of the night, while my wife is fast asleep, and finish it all myself, picking at the crispy, cheesy bits around the edges of the casserole in the wee hours of the morning, and leaving just enough to hint at the glory that it once contained. What, dear? What's the lingering aroma of garlic and thyme in the air, you ask? Ah, well, you snooze, you lose, hon.

I don't know how much longer this type of behavior is going to be tolerated.