Pour the mixture into the prepared dish. Sprinkle the remaining 1 1/2 cups cheddar cheese, 1/2 cup Gruyère (or 1/4 cup Pecorino Romano), and the breadcrumbs over the top. Bake until golden brown, about 30 minutes (though we needed a bit more time to get it brown, but your oven may vary). Transfer the dish to a wire rack for 5 minutes; serve.