Potato and Rutabaga Gratin

Heat oven to 425°. Lightly butter a 9-by-13-inch baking dish.

Heat butter and oil in a large saucepan over medium-high heat; add garlic and onion, and cook, stirring often, until soft, about 6 minutes. Stir in flour, and cook until smooth, about 1 minute. Add milk and cream, and stir until smooth. Add potatoes, rutabagas, and 2 tsp. thyme, and bring mixture to a boil; cook, stirring often, until vegetables are slightly tender and broken apart, about 5 minutes. Stir in half the cheese and salt and pepper, and then transfer to prepared baking dish; top with remaining cheese and bake until golden brown and bubbling, about 25 minutes. Sprinkle with remaining thyme before serving.