1 (3-pound) head cauliflower, cut into large florets
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
1/2 cup freshly grated Parmesan
1/4 cup fresh bread crumbs
Preheat the oven to 375 degrees F.
Cook the cauliflower florets in a large pot of boiling salted water for 5 to 6 minutes, until tender but still firm. Drain.
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. Off the heat, 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 rest 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.