- Preheat the oven to 375°F.
- 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.
- 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.
- In a small saucepan, heat the milk over medium until small bubbles appear around the edges, then shut off the heat.
- 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.
- 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.
- 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.
- 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.