Halve and destone the plums, then place in a bowl with the icing sugar. Strip in the rosemary leaves, then leave to macerate for 30 minutes. Preheat the oven to 350°F.
For the frangipane, cream the butter and sugar together in a large, deep bowl using the bamix® with beater attachment, then beat in the eggs. Finely grate in the orange and lemon zest, then fold through with the almond extract, ground almonds and flour.
Roll out the pastry onto a floured surface to fit a 10in x 14in lined baking tray, going up the sides slightly. Bake in the middle of the oven for 15 minutes, or until lightly golden. Remove the tray and lower the heat to 325°F.
Spread the frangipane over the pastry and stud with the plums and flaked almonds. Bake for 30 minutes or until sticky and golden, then remove and leave to cool slightly.
Combine the icing ingredients in a bowl and mix until smooth, then drizzle over the tart and slice into pieces.