For the bottom apricot coulis, mix together the Jell-O, water, apricot jam, and orange blossom water. Strain the glaze through a mesh strainer.
Divide the apricot glaze among the serving glasses, covering about one-quarter of each glass.
Slice a few apricots in half and place two pieces around the rim of each glass.
Refrigerate for 15 minutes to allow the apricot coulis to set slightly.
To make the pudding, crush the mastic gum, if using, with a mortar and pestle.
Place the milk, sugar, and dissolved cornstarch in a large stockpot over medium heat.
While whisking, bring to a boil, then reduce the heat to low and continue whisking for a few seconds. Don’t overboil, otherwise the cornstarch may lose some of its thickening power and effectiveness.
The mixture at this stage should have the consistency of custard. Note that it will thicken further as it cools. Add the crushed mastic and orange blossom water, then give it a quick stir.
Remove from the heat and gently spoon the mixture into the ramekins, fully covering the apricot at the bottom. Set aside to cool completely.
Cover the ramekins with plastic wrap and chill for at least 4 hours.
Garnish with some crushed pistachios.