Crunchy Fried Potato Gratin Cubes
Lior Lev Sercarz, from "Mastering Spice" by Lior Lev Sercarz
5 whole dried bay leaves
2 teaspoons dried rosemary (2 grams)
1 tablespoon dried thyme (2 grams)
1 tablespoon dried garlic slices (5 grams)
1⁄2 teaspoon black peppercorns, preferably Tellicherry (2 grams)
Finely grind together the bay leaves, rosemary, thyme, garlic, and peppercorns.
Don't feel like blending? Substitute 2 tablespoons of finely ground Ayala N.16
1 tablespoon salted butter, plus more for the baking dish
2 medium yellow onions, finely diced
3⁄4 cup whole milk
3⁄4 cup heavy cream
31⁄2 pounds Yukon Gold potatoes (about 5 large), scrubbed
21⁄2 ounces Gruyère cheese
Preheat the oven to 350°F. Line a half sheet pan with parchment paper. Butter a 2-quart (8 × 11-inch) baking dish and set it on the lined half sheet pan.
Melt the butter in a large saucepan over medium heat. Add the onions and stir to coat with the butter, then add the spice blend and season with salt. Stir well, cover, and reduce the heat to medium-low. Cook, stirring occasionally, until the onions are soft, about 3 minutes.
Add the milk and cream, stir well, and reduce the heat to maintain a low simmer. Cook for about 5 minutes to infuse the liquids with the spices. Season very generously with salt. Taste it; it should taste very salty—the potatoes will remain unseasoned, so this mixture provides all the flavor.
While the cream simmers, cut the potatoes into 1⁄16-inch-thick slices using a mandoline, food processor fitted with a slicing blade, or a very sharp knife. If you’re still slicing after the cream has simmered for 5 minutes, simply turn the heat to low to keep it warm. Put the sliced potatoes in a very large bowl and pour the cream over them. Gently fold to ensure all the slices are coated. Let sit for at least 5 minutes to allow the potatoes to absorb the liquid and soften slightly.
To assemble the gratin, you can either go more freestyle, dumping all the potatoes and the cream into the prepared baking dish and pressing them down, or shingle the potato slices, along with any onions clinging to them, laying them in the pan in rows and overlapping them slightly. After each layer, press the potatoes flat. (I prefer the latter method because it ensures that the gratin will hold together.) Pour the spiced cream remaining in the bowl evenly over the potatoes. Set aside while you grate the Gruyère. Sprinkle the Gruyère evenly on top.
Bake until the top is dark golden brown, the liquid is bubbling, and the potatoes are tender, about 11⁄2 hours. If you insert a metal cake tester or thin-bladed knife into the center, it should slide through easily.
Cool completely. Set it on a clean half sheet pan and cover tightly with plastic wrap. Find a pan that fits snugly on top and put heavy cans or a small cast-iron skillet in it to weigh it down and compress the gratin. Refrigerate until very cold and set, preferably overnight or for up to 2 days. Unwrap and cut the gratin into 2-inch squares. Transfer the squares to a cutting board and trim any curved sides to form straight edges. (You can snack on the scraps!)
Heat a large cast-iron or nonstick skillet over medium heat until hot. Add 1 tablespoon olive oil and 1 tablespoon salted butter. As soon as the butter melts, add as many squares as will fit comfortably, cut-side down. Fry until golden brown on the bottom, about 2 minutes. Flip and brown another cut side. Repeat until all the cut sides are browned, 6 to 8 minutes total, adding more oil and butter as needed. Repeat with the remaining gratin. Drain on paper towels and serve hot.