Potato and Rutabaga Gratin

• 4 Tbsp unsalted butter
• 2 Tbsp olive oil
• 4 cloves garlic, finely chopped
• 1 medium onion, thinly sliced
• 1/4 cup flour
• 2 cups milk
• 1 cup cream
• 1 lb potatoes, peeled and very thinly sliced
• 1 lb rutabagas, peeled and very thinly sliced
• 1 Tbsp minced thyme leaves
• 2 cups (about 4-6 oz) grated Gruyére cheese, or other melty cheese
• kosher salt and freshly ground black pepper, to taste

1. Heat oven to 425°F. Heat butter and oil in 6-qt. saucepan over medium-high heat; add garlic and onion, and cook, stirring often, until soft, about 6 minutes.

2. Stir in flour, and cook until smooth, about 1 minute. Add milk and cream, and stir until smooth. Add potatoes, rutabagas, and 2 teaspoons thyme, and bring mixture to a boil; cook, stirring often, until vegetables are slightly tender and broken apart, about 5 minutes.

3. Add in half the cheese and salt and pepper, and transfer mixture to 9" × 13" baking dish; top with remaining cheese and bake until golden brown and bubbling, about 25 minutes. Sprinkle with remaining thyme before serving.