MAIN SPICE BLEND:
- 5 whole dried bay leaves
- 2 tablespoons dried rosemary (2 grams)
- 1 tablespoon dried thyme (2 grams)
- 1 tablespoon dried garlic slices (5 grams)
- ½ teaspoon black peppercorns, preferably Tellicherry (2 grams)
- 1 tablespoon salted butter, plus more for baking dish
- 2 medium yellow onions, finely diced
- Kosher salt
- ¾ cup whole milk
- ¾ cup heavy cream
- 3 ½ pounds Yukon gold potatoes (about 5 large), scrubbed
- 2 ½ ounces Gruyere cheese
Makes one 8 x 11-inch gratin, serves 8 to 12
*Finely grind together the bay leaves, rosemary, thyme, garlic, and peppercorns.
*Make ahead: The gratin can be refrigerated for up to 1 week.
1. To make the gratin: Preheat the oven to 350F. Line a half sheet pan with parchment paper. Butter a 2-quart (8 x 11-inch) baking dish and set it on the lined half sheet pan.
2. 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.
3. 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.
4. 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 the slices are coated. Let sit for 5 minutes to allow the potatoes to absorb the liquid and soften slightly.
5. 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 later 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 Gruyere. Sprinkle the Gruyere evenly on top.
6. Bake until the top is dark golden brown, the liquid is bubbling, and the potatoes are tender, about 1 ½ hours. If you insert a metal cake tester or thin-bladed knife into the center, it should slide through easily.
7. 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 on it to weigh it down and compress the gratin. Refrigerate until very cold and set, preferably overnight and up to 2 days.
8. 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).
9. 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-sides 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.
10. Drain on paper towels and serve hot.