Preparation

• 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 well.
  Add the broccoli to the boiling water. Cook until just tender, about 5 minutes. Drain the broccoli in a colander. Rinse under cold running water to cool. Drain well.
  Meanwhile, melt the butter in a medium saucepan over low heat. Add the flour, stirring constantly with a whisk 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 1/2 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 and broccoli on top and then spread the rest of the sauce evenly on top. Combine the bread crumbs with the remaining 1/4 cup of cheese and sprinkle on top. Melt the remaining 2 tablespoons of butter and drizzle over the gratin. Bake for 25 to 30 minutes, until the top is browned. Serve hot.