The caramelized onions in this creamy potato gratin draw out the sweet warmth of the pinch of saffron that makes this dish golden. A topping of shredded Gruyère cheese adds to the decadence.
In a saucepan, heat cream, 1 teaspoon salt, 1/8 teaspoon pepper, saffron, and nutmeg over medium until bubbling around edges. Remove from heat; let stand 20 minutes.
Meanwhile, heat butter in a medium skillet over medium until bubbling. Add onion, season generously with salt, and cook, stirring occasionally, until softened, about 10 minutes. Reduce heat to medium-low and continue cooking until deep golden brown, about 15 minutes. (If mixture is becoming too dark, add water, 1 teaspoon at a time, and scrape up any browned bits; you should have about 1/2 cup caramelized onion.)
Preheat oven to 300°F. Brush a 2-quart baking dish with butter. Peel and thinly slice potatoes 1/8-inch thick. Shingle half of potatoes in bottom of dish. Top with caramelized onion and half of cheese. Shingle remaining potatoes over top. Pour cream mixture over potatoes, and top with remaining cheese. Bake until potatoes are fork-tender and top is bubbling and golden, about 1 hour and 25 minutes. Let stand at least 15 minutes before serving.
Use a mandoline slicer to cut thin, even slices of potato.