This gratin is kind of French. It has the requisite butter, milk, and cream, but I use onions too—a debated addition—and, of course, spices. To infuse each layer of potatoes with spices and salt, I simmer both in the cream rather than sprinkling them on the potatoes.
The mix of bay leaves, rosemary, thyme, garlic, and pepper tastes distinctly French. The resulting gratin ends up versatile enough for any menu.
Rosemary & Thyme Potato Gratin with Bay Leaves
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 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.