Brush each pastry case with beaten egg and divide the labne into each pastry case. There should be 1 heaped tablespoon per case. Arrange the prepared plums in each pastry case, sitting them vertically with the cut-side up. You should be using about 9 halves per case. Press the plums down into the labne, sprinkle each tart with 1 teaspoon of caster sugar, place on a baking tray and bake for approximately 40 minutes, or until set and caramelised.