In a medium saucepan, bring the milk to a gentle simmer, stirring it continuously to avoid scalding. Add the sugar, crushed mastic, and cornstarch or rice-flour slurry and stir to dissolve them.
Reduce the heat to low and continue stirring until the mixture thickens, then stir in the rose and orange-blossom waters. Remove the pudding from the heat. Immediately pour into individual bowls