Bring the juice (or jam and water) to a gentle simmer on medium heat, stirring it continuously. Add the sugar, crushed mastic, and cornstarch slurry and stir to dissolve them.
Reduce the heat to low and continue stirring until the mixture thickens. If you are serving the fruit pudding on its own, pour it into individual bowls and allow it to cool. If you are preparing it for parfaits, o