For the pudding: With a mortar and pestle, crush the mastic with a pinch of sugar (if used) and add it to large pot.
Pour the milk into the pot, add the semolina and butter.
Place the pan over high heat, whisking constantly until the mixture totally thickens and boils for 1 minute. Pour over a large casserole or ovenproof dish. Cover with parchment paper to prevent from forming a skin. Allow to cool down completely.
To make the sugar syrup (atter): Place the water, sugar and lemon juice in a saucepan over medium heat. Stir the mixture with a wooden spoon to dissolve the sugar.
One it boils reduce 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 totally thickens up.
Transfer layali Lubnan to a casserole or individual ramekins. Leave to cool down completely.
The final touch: Spread the ashta cream covering the casserole or the ramekins, garnish with raw shelled crushed pistachios and orange petals jam. Cover with clingfilm and refrigerate for a minimum of 6 hours or overnight.
Serve with orange blossom scented sugar syrup. Enjoy.