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The Food Lab: Hasselback Potato Gratin (These Might be the Best Potatoes Ever)
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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 un-cited online references, the dish was created at the Hasselback restaurant in Stockholm some time in the 18th century (this was way before the internet started bacon-ing everything).
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 each ridge, and baking them until 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!"
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.
The dish starts out just like most potato gratins: 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 Japanese 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.
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)
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.
That cream not only adds flavor and moisture, but it also keeps the potato slices from sticking together as they cook. Without it, starches released by slicing the potatoes swell up, sucking up extra moisture and turning the interior of the casserole gluey and starchy instead of creamy and tender.
Here's where we turn things on their
head side: rather thank stacking sliced potatoes horizontally like in a traditional gratin, I pack them into a greased casserole 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.
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.
So what's the trick to getting the top crisp instead of leathery? Well, the coating of fat helps, for starters. It prevent 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 tender, followed by a good cook uncovered to dry them and brown them.
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.
A final layer of cheese added halfway through the uncovered portion of the cook adds a layer of flavor to the final casserole.
During the final 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.
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 a 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 only make this 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 dish in the wee hours of the morning, 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.
About the author: J. Kenji Lopez-Alt is the Chief Creative Officer of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.