Cauliflower Gratin

1 (3-pound) head cauliflower, cut into large florets
Kosher salt
4 tablespoons (1/2 stick) unsalted butter, divided
3 tablespoons all-purpose flour
2 cups hot milk
1/2 teaspoon freshly ground black pepper
1/4 teaspoon grated nutmeg
3/4 cup freshly grated Gruyere, divided (can be replaced with cheddar)
1/2 cup freshly grated Parmesan
1/4 cup fresh bread crumbs
 
Preheat the oven to 375 degrees F.
Bring water to a boil in a large pot over high heat, and add the cauliflower florets. Cook the cauliflower for 5 to 6 minutes, until it’s tender but still firm. Drain, and set aside.
Meanwhile, melt 2 tablespoons of the butter in a medium saucepan over low heat. Add the flour, stirring constantly with a wooden spoon for 2 minutes. Pour the hot milk into the butter-flour mixture, and stir until it comes to a boil. Boil, whisking constantly, for 1 minute, or until thickened.
Take the pan off the heat, and add 1 teaspoon of salt, the pepper, nutmeg, 1/2 cup of the Gruyere, and the Parmesan.
Pour 1/3 of the sauce on the bottom of an 8 by 11 by 2-inch baking dish. Place the drained cauliflower on top, and then spread the remaining 2/3 of the sauce evenly on top.
Combine the bread crumbs with the remaining 1/4 cup of Gruyere and sprinkle on top.
Melt the remaining 2 tablespoons of butter, and drizzle over the gratin. Sprinkle with salt and pepper. Bake for 25 to 30 minutes, until the top is browned. Serve hot or at room temperature.
Serves 4-6.