1) For the compote: Combine all of the ingredients, except for the tapioca, in a saucepan over medium heat to extract the juices, and then cook until the rhubarb is al dente. Add the starch and simmer until the compote thickens. Remove from the heat and transfer to a pan to cool. Serve with Streusel.
2) For the streusel: Preheat oven to 150C/Gas 2.
3) Cream together the butter and sugar in a mixing bowl. Add flour, combine until pebbly.
4) Press together in your hand. Break off pieces and bake until golden brown