1. Preheat the oven to 375°F.
  2. Set a large pot of salted water to boil. Add the potatoes and cook until fork-tender, about 10 minutes. Using a slotted spoon, transfer to a colander set over a bowl.
  3. To the same water, add the broccoli and cook until tender, about 5 minutes. Place the colander into the sink and drain the broccoli over the top.
  4. In a small saucepan, heat the milk over medium until small bubbles appear around the edges, then shut off the heat.
  5. In a large skillet with high sides, melt three tablespoons of the Kerrygold Salted Butter over low heat. Whisk in the flour to form a paste and cook, constantly whisking, for one minute.
  6. Gradually whisk in the warm milk. Add 1 cup of Kerrygold Dubliner Cheese and the nutmeg and whisk until the cheese is melted and the sauce is smooth. Turn off the heat.
  7. Season the sauce to taste with salt and pepper, then add the potato-broccoli mixture. Mix well to combine, then transfer to a 9 x 13-inch or 3 to 4 quart casserole dish.
  8. Sprinkle the remaining 1/4 cup of Kerrygold Dubliner Cheese, bread crumbs, and thyme leaves and bake until bubbly along the edges and golden brown on top, about 15 minutes.