Bring a saucepan of salted water to a boil. Add the cauliflower and cook until tender, about 3 minutes. Drain.
In the meantime, melt the butter in a saucepan over medium heat. Add the flour and cook for a few minutes. Whisk in the water. Season with salt, pepper, and nutmeg. Allow the sauce to cook for a few minutes further. Stir occasionally.
Whisk half the Parmesancheese and all the Gruyère into the sauce, and cook a bit more.
Divide the cauliflower between individual gratin dishes and spoon the sauce over the top. Then sprinkle the remaining Parmesancheese, along with the bread crumbs, over the top. Finally, sprinkle a half tablespoon of melted butter over each portion.
Place the dishes on a baking sheet and bake until the tops are golden, about 10 minutes.