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