Potato and Celery-Root Gratin

Preheat oven to 400 degrees F. In a medium saucepan over medium-high heat, bring heavy cream, milk, onion halves, and garlic to a boil. Remove from heat and let steep for 30 minutes.

Meanwhile, generously butter an 8-inch square baking dish and one side of a piece of foil large enough to seal over the dish. Using a sharp knife, peel, halve, and thinly slice celery root; cut slices in half. Peel and thinly slice potatoes. In prepared dish, arrange a layer of celery-root slices followed by a layer of potato slices; season to taste with salt and pepper. Repeat.

Remove onion and garlic from cream mixture and discard. Pour mixture over casserole. Cover pan with prepared foil and transfer to oven. Bake for 40 minutes.

Remove foil and sprinkle top of casserole with cheese. Return to oven and bake until bubbly and golden, about 15 minutes more. Transfer to a wire rack and allow to rest 15 minutes before serving.