Cheese gratin with pearl barley

Ingredients:

• 110g unsalted butter, plus extra for greasing the pan
• 2 cups pearl barley
• 170 g grated Gruyere (about 2 cups)
• 2 tbsp. grated mozzarella
• 1 tbsp. grated parmesan
• 1.5 cups panko breadcrumbs
• 1 large onion, cut into 1/4-inch pieces (about 2 cups)
• 2 large cloves garlic, minced
• 1.5 tsp finely chopped fresh thyme
• 1/4 cup premium flour
• 4 cups whole milk

Preparation:

1. Position the oven rack in the middle position and preheat the oven to 200°C. Lightly grease a 3-quart baking dish with butter and set aside.
2. Place a small saucepan of water, add a little salt, and bring to a boil over medium heat. Add the pearl barley and cook until tender but still slightly firm, about 20 minutes (it will finish cooking in the oven). Drain and rinse the pearl barley.
3. Meanwhile, place 2 tablespoons of butter in a medium microwave-safe bowl and melt in the microwave for about 30 seconds. In another medium bowl, combine the Gruyere, mozzarella, and Parmesan. Add panko breadcrumbs, 1 cup of cheese mixture, and 0.5 teaspoon of salt into a bowl with melted butter. Using your hands, mix the butter into the cheese and breadcrumbs. Set aside.
4. In a large, wide saucepan, melt the remaining 6 tablespoons butter over medium-high heat. Add the onion and a large pinch of salt and cook, stirring occasionally, until the onion is golden brown, 6 to 8 minutes. The butter will foam and brown spots will appear, but this is normal (the browned butter will impart more flavor to the finished dish). Reduce the heat to medium, add the garlic and thyme, and stir constantly until the garlic is softened, about 1 minute. Add the flour and cook, stirring constantly, until the onion mixture forms a thin paste, about 1 minute.
5. Stir in the milk and 1 teaspoon each of salt and black pepper and bring to a boil over medium heat. Reduce the heat and simmer, stirring occasionally, until the sauce has thickened into a thin gravy, about 10 minutes. Remove from the heat and stir in the cheese mixture and cooked barley. Season with salt and pepper to taste.
6. Place the mixture in a greased baking dish. Sprinkle evenly with the cheese breadcrumbs and bake until golden brown, about 30 minutes. Let the casserole rest for 10 minutes and serve.