Rutabaga Gratin

Bahan-bahan (6 porsi)

• 3 ½ to 4 pounds rutabaga (cubed)
• ½ cup chopped onion
• 3 tablespoons butter
• 3 tablespoons all-purpose flour
• ½ teaspoon dried thyme leaves
• 2 cups milk
• 1 cup shredded sharp cheddar cheese
• ½ cup shredded Gruyere cheese