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. Melt remaining 2 Tbsp. butter, combine with bread crumbs, remaining ¼ cup Parmesan, and thyme; season with salt and black pepper. Sprinkle crumbs over gratin and bake until crumbs are golden brown and sauce is bubbly, 25–30 minutes. Let gratin rest 10 minutes before serving.