Heat oven to 220C/200C fan/gas 7. Halve the pastry, then roll out each half on a lightly floured surface to about the thickness of a £1 coin. In the centre of 1 pastry sheet, spread the jelly into a circle roughly the same size as the cheese. Sit the cheese on top.
Brush the pastry around the cheese with some beaten egg. Lift the second sheet of pastry on top and gently press over the cheese, squeezing out the air as you go, then seal it all the way around – press down with your fingers over all the excess pastry to seal really well. Use scissors to cut away excess pastry, leaving about a 4cm border around the cheese. Use the prongs of a fork to press the edges – this looks pretty and seals the pastry even more. Lift onto a baking tray lined with baking parchment. Brush all over with egg, then add a sesame-seed star, if you like.
Bake for 20-25 mins until the pastry is brown, crisp and well risen around the edges – if your star is getting too dark, cover with a square of foil after 20 mins. Sit for 5 mins before cutting and catching the cheesy centre as it oozes out.