Cheesy Barley Gratin

Name: Cheesy Barley Gratin
Ingredients: 8 tablespoons (1 stick) unsalted butter, plus more for greasing the baking dish; Kosher salt and freshly ground black pepper; 2 cups pearled barley; 6 ounces Gruyere, shredded (about 2 cups); 2 cups shredded mozzarella; 1 cup grated Parmesan; 1 1/2 cups panko breadcrumbs; 1 large yellow onion, chopped into 1/4-inch pieces (about 2 cups); 2 large cloves garlic, finely minced; 1 1/2 teaspoons finely-chopped fresh thyme; 1/4 cup all-purpose flour; 4 cups whole milk