Cover tapioca with cold water and soak overnight or until tapioca has absorbed all the water. Add the boiling water and salt and cook until the mixture comes to the boiling point. Put over water in the top of a double boiler until it becomes transparent.
Place rhubarb in a well-buttered casserole or baking dish, cover with sugar and nutmeg, pour tapioca over and bake 25 to 30 minutes in a preheated 350° oven.
Serve hot or cold with lemon sauce, as follows: