Line a 9" pan with wax paper if using a springform pan; if using a glass serving dish, there is no need to line it. Cover the bottom with all the pieces of bread or lady fingers.
Place the sugar and water in a saucepan and bring to a simmer. Stir gently from time to time, add the lemon juice; simmer for 10 minutes or so, then add the rose and orange blossom water and turn off the heat. Cool; moisten the bread with a few tablespoons of syrup. Pour the remaining syrup in a small container to serve alongside the dessert.
In a saucepan, pour 3 cups of milk and the sugar over medium heat; when steam appears, add the remaining milk and cornstarch mixture and stir continuously till thick.
Add the mastic powder and the rose and orange blossom, stirring to combine, in the last few seconds of cooking.
Set aside to cool one minute, then gently pour over the bread in the pan.
Cool the custard then refrigerate
Garnish with ground pistachios.
Serve cold with additional syrup on the side.
Enjoy with a cup of Arabic coffee