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1
Position an oven rack in the center of the oven and preheat it to 400 degrees F. Lightly grease a 3-quart baking dish with butter and set aside.
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2
Fill a small pot with lightly salted water and bring to a boil over medium-high heat.
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3
Add the barley and boil until just cooked through but still slightly chewy, about 20 minutes (it will continue to cook in the oven).
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4
Drain and rinse.
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5
Meanwhile, put 2 tablespoons of butter in a medium microwave-safe bowl and microwave until melted, about 30 seconds.
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6
Combine the Gruyere, mozzarella and parmesan in another medium bowl.
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7
Add the panko, 1 cup of the cheese mixture and 1/2 teaspoon salt to the bowl with the melted butter.
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8
Use your hands to work the butter into the cheese and breadcrumbs.
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9
Set aside.
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10
Melt the remaining 6 tablespoons of butter in a wide large pot over medium-high heat.
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11
Add the onions and a large pinch of salt and cook, stirring occasionally, until the onions are lightly browned, 6 to 8 minutes.
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12
(The butter will start to foam and brown specks will form, but that is okay; this will add a lot of flavor to the finished dish.)
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13
Reduce the heat to medium, add the garlic and thyme and stir constantly until the garlic is just softened, about 1 minute.
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14
Stir in the flour and cook, stirring constantly, until a thin paste forms around the onion mixture, about 1 minute.
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15
Whisk in the milk and 1 teaspoon each salt and pepper and bring to a boil over medium-high heat.
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16
Reduce to a simmer, stirring occasionally until the sauce is a thin gravy, about 10 minutes.
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17
Remove from the heat and stir in the shredded cheese mixture and cooked barley.
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18
Season to taste with additional salt and pepper.
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19
Spoon into the greased baking dish.
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20
Sprinkle the reserved cheesy breadcrumbs evenly on top of the barley and bake until bubbly and the top is golden brown, about 30 minutes.
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21
Let stand 10 minutes before serving.
