How to Make It
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.