Pre-heat the oven to 200C/180C fan/Gas Mark 6 and line 2 baking trays with baking parchment.
Mix all the ingredients for the filling together in a bowl.
Unroll the sheet of pastry on a lightly floured work surface and roll out by another 2.5 cm all round.
Using a 9 cm square pastry cutter, cut out squares from the pastry, you should get around 12 from the sheet. Wrap the pastry trimmings in clingfilm and place in the fridge for re-rolling to make more squares later.
Place a heaped tablespoonful of the filling in the centre of each square then brush beaten egg around the border with a pastry brush and fold over to make a triangle.
Seal the triangles by pressing down on the edges with the tines of a fork and at teh same time making a nice pattern.
Place on the lined trays.
Re-roll and repeat with the chilled pastry trimmings.
Brush each of the triangles liberally with beaten egg (this alsohelps to prevent the filling from leaking out while baking and sprinkle over the sesame and nigella seeds.
Bake for 25-30 minutes until puffed up and golden. Enjoy hot or cold.
I didn't have a square cutter, so used a round one. I also didn't have any nigella seeds so used more sesame seeds.