Place the mastic gum in a small mortar and pestle with about 1/2 teaspoon of the granulated sugar.
Crush into a fine powder and set aside.
In a small bowl, whisk together the cornstarch and water until dissolved and smooth. Set aside.
In a medium saucepan, whisk together the remaining sugar, milk, and cream over medium heat.
Once the liquid is starting to boil and all the sugar has dissolved, reduce heat to a simmer.
Slowly pour in the cornstarch mixture while whisking, then the ground mastic gum. Continue to simmer, while whisking, until the mixture thickens.
Remove from heat and whisk in the orange blossom water.
Allow to cool slightly, then transfer to a bowl, cover and refrigerate until well chilled, 4 hours to overnight.
Pour into prepared ice cream maker and churn according to manufacturer's instructions.
Either serve straight from the ice cream maker with a generous covering of chopped pistachios or transfer to a freezer safe bowl and freeze for up to a month.