Preheat the oven to 375 degrees F. In a large pot of boiling salted water, cook cauliflower 5 to 6 minutes or until tender but still firm. Drain.
Melt 2 tablespoons of the butter in a medium saucepan over low heat. Add the flour, stirring constantly for 2 minutes to make a white roux. Pour the hot milk into the roux mixture and whisk until it comes to a boil. Continue whisking constantly, for 1 minute, or until thickened. Remove from heat. Stir in 1 teaspoon of salt, pepper, nutmeg, 1/2 cup of the Gruyere, and the Parmesan.
Pour 1/3 of the sauce on the bottom of an baking dish. Add the cooked cauliflower and top with remaining sauce.
Combine the bread crumbs with the remaining 1/4 cup of Gruyere and sprinkle on top. Melt remaining butter and drizzle over the gratin.
Bake for 25 to 30 minutes, until the top is browned. Serve hot or at room temperature.