Preheat the oven to 200°C/fan 180°C/gas mark 6. Heat the oil in a small pan, add the onion and cook for 3-4 mins or until soft.
Blot prunes dry with absorbent kitchen roll, then roughly chop the flesh.
Mix the sausage meat, onion and prunes until well combined, season to taste.
Lay the pastry on a work surface, then cut in half length-ways.
Lightly dust your hands with flour. Divide sausage meat mixture in two, then roll out each half to form a long sausage shape. Lay a sausage meat roll length-ways on each piece of pastry.
Brush one of the longer edges of the pastry with the beaten egg then fold the pastry over the filling, and seal by pressing gently on each roll.
Turn the roll over so the seal is on the underside. Cut each strip into 8 rolls.
Brush the pastry with remaining beaten egg, then sprinkle with poppy or sesame seeds.
Place the rolls on a baking tray and cook for 20-30 minutes or until crisp and golden.