Preheat oven to 350F. Mix sugar, tapioca and cinnamon together in a medium mixing bowl. Toss them with diced rhubarb and pour everything into a greased 8 x 8-1/2 inch glass baking dish. Mix the biscuit mix, milk, sugar and egg together thoroughly and spread it on top of the rhubarb mixture. Bake it at 350F for 45 minutes or until a toothpick inserted into the biscuit top comes out clean. Serve it warm and place each piece upside down on a plate, then add topping.
To make topping:Pour cold milk into a small heavy saucepan. Add the butter and flour, stirring them with a wooden spoon until the flour has dissolved. Cook it over low heat, stirring constantly until the sauce has thickened. Add the allspice and sugar. Cool the topping until it is warm and pour it over each piece of the upside down cake.
Cajun Country -- New Iberia, Louisiana