- 2 tablespoons unsalted butter, plus more for the baking dish
- 1 medium onion, finely chopped
- 2 cups heavy cream
- 2 cups whole milk
- 1 bay leaf
- 1 teaspoon fresh thyme leaves
- 1/4 teaspoon ground nutmeg
- kosher salt and black pepper
- 4 pounds russet potatoes (8 medium), peeled and sliced 1/8 inch thick
- 6 ounces Gruyère or Cheddar, grated (about 1 1/2 cups)
- Heat oven to 375° F. Butter a 9-by-13-inch or some other shallow 3-quart baking dish. In a large saucepan or Dutch oven, melt the butter over medium-high heat. Add the onion and cook, stirring often, until softened, 5 to 7 minutes. Add the cream, milk, bay leaf, thyme, nutmeg, 2 teaspoons salt, and ½ teaspoon pepper and bring to a simmer.
- Add the potatoes to the saucepan and simmer, stirring occasionally, until tender, 30 to 35 minutes.
- Discard the bay leaf and transfer the potato mixture to the prepared baking dish; sprinkle with the Gruyère.
- Transfer to the top rack of the oven and bake until bubbling and golden brown, 20 to 25 minutes.