First, roll out the pastry on a lightly floured surface into a rectangle, roughly 32 x 20cm. Cut in half so you have 2 long rectangles, place on baking parchment on a baking sheet and chill in the fridge.
Put the potatoes in a large pan of salted water and bring to the boil. Reduce the heat, cook until just tender (about 5 mins), drain and leave to cool.
Meanwhile, remove the chorizo skins and put the meat in a large bowl of a food processor. Add the parsley, 1 tbsp nigella seeds and the cooled potatoes, and pulse everything together, being careful not to break up the potato chunks too much.
Heat oven to 200C/180C fan/gas 6. Take the pastry out the fridge and shape the chorizo mix into two long sausages. Put each in the middle of the pastry pieces and brush the edges of the pastry with the beaten egg. Bring the pastry together on one side to enclose the filling and seal by pressing the pastry with a fork. Cut the rolls into 9 pieces each (about 5cm long) and arrange on two baking trays. Brush with egg and scatter over the remaining nigella seeds. Bake for 25-30 mins until golden and cooked through. Leave to cool completely before packing for your picnic.