Parsnips lend a nutty sweetness to this classic holiday side dish. Assemble it the night before and then pop it in the oven an hour before dinnertime for easy entertaining.
1. Preheat oven to 350°F. Coat a 2-quart baking dish with cooking spray. Peel the potatoes and the parsnips. Using a mandoline, cut potatoes and parsnips into ⅛-inch-thick slices.
2. In a large heavy-bottomed pot, combine the half & half, milk, garlic powder, and thyme. Season with salt and pepper. Add the potatoes and parsnips and heat to a boil on high. Reduce heat and simmer 5‒6 min., until potatoes are almost tender and liquid has thickened. Remove from heat.
3. Spoon half of the mixture into the greased baking dish, spreading in an even layer. Top with half of the Gruyère. Repeat layering. Bake 50‒60 min., until golden brown. Let cool 10 min. before serving.
A food processor fitted with a slicing blade can also get the vegetables paper-thin in a flash.