Potato and Poblano Gratin

Preparation

1. Step 1

   Arrange one rack in the center of the oven and another one 5 inches from the heat source. Heat the broiler on high. Place the peppers on a rimmed baking sheet and broil on the top rack until deeply blistered, about 12 minutes. Turn with tongs and broil on the other side until deeply blistered, about 12 minutes more. 

2. Step 2

   Place the charred peppers in a large bowl; cover them with a plate and let them steam for 10 minutes to loosen the skins. On a cutting board, run the dull side of a paring knife across each chile to scrape off most of the blackened, papery skin. (Don’t worry if not all of it comes off — the remainder will add a nice smoky note.) Halve the peppers and remove the stems and seeds, then cut the peppers into 1½-inch pieces. Season with ½ teaspoon salt. Set aside.

3. Step 3

   Heat oven to 350 degrees. Grease a 13-by-9-inch baking dish or large gratin dish with 1 tablespoon butter.

4. Step 4

   Meanwhile, in a medium saucepan, combine the cream, coriander, cumin, jalapeño, garlic, thyme sprigs, remaining 2 tablespoons butter and 1¾ teaspoons salt. Bring to a gentle simmer over medium-low heat, whisking until the butter melts, about 4 minutes. (Be mindful not to let it simmer vigorously, as this may cause the cream to break.) Remove from heat and let steep while you prepare the potatoes.

5. Step 5

   Remove any eyes from the potatoes using the tip of a vegetable peeler or a paring knife. Slice the potatoes crosswise using a sharp knife or mandoline into ⅛-inch-thick slices. Divide them into four equal piles, saving the prettiest ones for the top layer. Arrange one-quarter of the potatoes in a single layer, fanning them out at an angle and overlapping them to cover the bottom of the dish, then sprinkle with black pepper. Scatter one-quarter of the chiles in an even layer. Repeat the layering process three more times. When you have the final layer of potatoes, tuck the remaining chiles between the potato slices.

6. Step 6

   Strain the cream into a liquid measuring cup, discarding the solids. Pour cream over potatoes. Set the baking dish on a foil-lined rimmed baking sheet and place it in the oven. Bake, uncovered, until potatoes are tender and the creamy sauce is bubbling around edges, 45 to 50 minutes. Remove from the oven and top with an even layer of Gruyère, then sprinkle with black pepper. Bake until the cheese is bubbling and the top of the gratin is lightly golden brown around the edges, 15 minutes more. 

7. Step 7

   Remove gratin from oven and let stand for 20 minutes before serving (for make-ahead instructions, see Tip).