For the pudding: With a mortar and pestle, crush the mastic with a pinch of sugar (if using), then add it to a large pot.
Pour the milk into the pot, then add the semolina and butter.
Place the pan over high heat and whisk constantly until the mixture thickens completely and comes to a boil. Continue boiling for 1 minute. Pour into a large casserole or ovenproof dish. Cover with parchment paper, pressing it directly onto the surface to prevent a skin from forming. Allow to cool completely.
To make the sugar syrup (atter): Place the water, sugar, and lemon juice in a saucepan over medium heat. Stir with a wooden spoon until the sugar has dissolved.
Once it boils, reduce the heat to low and simmer for 7 minutes. Do not cover the saucepan. Add the orange blossom water and simmer for 1 more minute, until it thickens completely.
Transfer the Layali Lubnan to a casserole dish or individual ramekins. Leave to cool completely.
The final touch: Spread the ashta cream evenly over the casserole or ramekins. Garnish with raw, shelled, crushed pistachios and orange blossom jam. Cover with cling film and refrigerate for a minimum of 6 hours or overnight.
Serve with orange blossom-scented sugar syrup. Enjoy!