Cook the cauliflower florets in a large pot of boiling salted water for 5 to 6 minutes, until tender but still firm.
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 in both milks 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, thyme, 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.