Preheat the oven to 175 °C. 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 the margarine in a saucepan over low heat. Add the flour, stirring constantly for 2 minutes. Pour water into the butter-flour mixture and stir until it comes to boil. Boil by whisking constantly until thickened. Off the heat, stir in the egg, cream, salt, pepper, nutmeg and water, if necessary. Place the drained cauliflower on a baking dish and spread the sauce on top. Sprinkle the cauliflower with bread crumbs. Bake for 20 to 30 minutes until the top is browned.