Potato and Rutabaga Gratin

4 tbsp. unsalted butter
2 tbsp. olive oil
4 cloves Garlic, finely chopped
1 medium red onion, thinly sliced
¼ cup flour
2 cups milk
1 cup heavy cream
1 lb. Red Dog Farm Potatoes, peeled and very thinly sliced
1 lb. Red Dog Farm Rutabagas, peeled and very thinly sliced
1 tbsp. minced thyme leaves
2 cups (about 4 oz.) grated Gruyére cheese
Kosher salt and freshly ground black pepper, to taste

Heat oven to 425°. Heat butter and oil in a 6-qt. 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 a 9″ × 13″ baking dish; top with remaining cheese and bake until golden brown and bubbling, about 25 minutes. Sprinkle with remaining thyme before serving.