Break the cauliflower into very large florets, trimming the stems and removing the leaves. Steam or boil the cauliflower until partially cooked. Transfer the par cooked cauliflower to a greased baking dish.
In a saucepan melt 1/4 cup of the butter over medium low heat. Add the finely diced onion and cook slowly until the onion is very tender and softened. Whisk in the flour to make a roux. Cook the roux for 2 to 3 minutes over medium low heat.
Begin adding the milk, half a cup at a time whisking to prevent lumps. Once all the milk has been added, raise the temperature to medium high and bring to a boil. Add the dry mustard, nutmeg, kosher salt, and pepper. Cook for 2 to 3 minutes until thickened. Remove from the heat and add 3/4 cup each of the cheeses. Whisk until the cheese is melted.
Preheat the oven to 400 deg F. Pour the cheese sauce over the cauliflower in the baking dish. Melt the remaining tablespoon of butter. In a small bowl combine the breadcrumbs, melted butter, remaining 1/4 cups of cheese, and chopped parsley. Sprinkle the crumbs evenly over the cauliflower.
Bake for 25 to 30 minutes until the sauce is bubbly, crumbs are browned, and the cauliflower is tender. Enjoy!