It’s important that the potatoes are sliced very thin, no more than 1/8” thick. I use a mandoline, but you can also use your food processor with the blade attachment. These are great to serve on Easter with a glazed ham or a leg of lamb.
2 tablespoons unsalted butter
1 medium onion, finely chopped (about 1 cup)
1 leek, trimmed, thinly sliced, and rinsed well
2 medium cloves garlic, minced or pressed through garlic press (about 2 teaspoons)
1 tablespoon chopped fresh thyme leaves
1/8 teaspoon freshly grated nutmeg
1 teaspoons table salt
1/2 teaspoon ground black pepper
2 pounds russet potatoes (about 4-5 medium), peeled and sliced thin
8 ounces celery root (about 1/2 medium), peeled and sliced thin
1 cup low-sodium chicken broth
1 cup heavy cream
2 bay leaves
4 ounces grated cheddar cheese or gruyere, shredded (1 cup)
1. Adjust oven rack to middle position; heat oven to 425 degrees.
2. Melt butter in large Dutch oven over medium-high heat until foaming subsides. Add onion and leek and cook, stirring occasionally, until soft and lightly browned, about 4 minutes. Add garlic, thyme, nutmeg, salt, and pepper; cook until fragrant, about 30 seconds. Add celery root, chicken broth, cream, and bay leaves and bring to simmer. Cover, reduce heat to medium-low, and simmer for 5 minutes. Add potatoes, bring mixture back to simmer, cover, and cook until potatoes are almost tender (paring knife can be slipped into and out of potato slice with some resistance), about 10 minutes. Discard bay leaves.
3. Transfer mixture to 8-inch-square baking dish (or other 1 1/2-quart gratin dish) and press to an even layer; sprinkle evenly with cheese. Bake until cream is bubbling around edges and top is golden brown, about 15 minutes. Cool 10 minutes before serving.
To Make Ahead: Once the scalloped potatoes have been transferred to the baking dish and pressed into an even layer in step 3, they can be wrapped in plastic wrap and refrigerated for up to 24 hours. When ready to bake, add the cheese, cover with foil, and bake in a 400-degree oven until the mixture is hot and bubbling, about 45 minutes. Remove the foil and cook until the cheddar begins to brown, about 30 minutes longer. Let cool for 10 minutes before serving.