8 tablespoons butter, plus more for greasing the baking dish
salt and freshly ground black pepper
2 cups pearled barley
6 ounces Kerrygold Irish Aged Cheddar, shredded (about 2 cups)
2 cups shredded mozzarella
1 cup grated Parmesan
1 1/2 cups breadcrumbs
1 large 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 plain flour
4 cups whole milk
Position an oven rack in the center of the oven and preheat it to 400 degrees F. Lightly grease a baking dish with butter and set aside.
Fill a small pot with lightly salted water and bring to a boil over medium-high heat. Add the barley and boil until just cooked through but still slightly chewy, about 20 minutes (it will continue to cook in the oven). Drain and rinse.
Meanwhile, put 2 tablespoons of butter in a medium microwave-safe bowl and microwave until melted, about 30 seconds. Combine the aged cheddar, mozzarella and parmesan in another medium bowl. Add the breadcrumbs, 1 cup of the cheese mixture and 1/2 teaspoon salt to the bowl with the melted butter. Use your hands to work the butter into the cheese and breadcrumbs. Set aside.
Melt the remaining 6 tablespoons of butter in a wide large pot over medium-high heat. Add the onions and a large pinch of salt and cook, stirring occasionally, until the onions are lightly browned, 6 to 8 minutes. Reduce the heat to medium, add the garlic and thyme and stir constantly until the garlic is just softened, about 1 minute. Stir in the flour and cook, stirring constantly, until a thin paste forms around the onion mixture, about 1 minute. Whisk in the milk and 1 teaspoon each salt and pepper and bring to a boil over medium-high heat. Reduce to a simmer, stirring occasionally until the sauce is a thin gravy, about 10 minutes. Remove from the heat and stir in the shredded cheese mixture and cooked barley. Season to taste with additional salt and pepper. Spoon into the greased baking dish. Sprinkle the reserved cheesy breadcrumbs evenly on top of the barley and bake until bubbly and the top is golden brown, about 30 minutes. Let stand 10 minutes before serving.