Heat frying pan over medium heat. Melt ½ cup of the butter; add breadcrumbs and fry, stirring, until golden brown. Set aside.
In a large pot, combine milk, sugar, semolina and mastiha and cook over low heat, stirring frequently, until creamy. Stir in orange-blossom water.
Rub a shallow oven dish with the remaining 2 tbsp butter, then sprinkle over half of the breadcrumbs. Sprinkle cheese over breadcrumbs, then top with an even layer of the semolina cream. Top with the rest of the breadcrumbs.
Bake until light golden and cheese is melted, about 20 minutes. Serve hot, drizzled with sugar syrup and sprinkled with ground pistachios.
Notes
To make sugar syrup, boil 3 cups granulated sugar with 1½ cups water until sugar is dissolved. Remove from heat and let cool completely.