Place the flour and butter in a food processor with seasoning and pulse until it resembles fine breadcrumbs. Beat one of the eggs with 75ml cold water and the vinegar, then tip it into the flour mixture. Pulse again until it just begins to come together, then tip into a bowl and knead briefly to form a smooth dough. Shape this into a disc, then wrap in clingfilm and chill for 1 hour.
Heat the oil in a medium frying pan over a medium/high heat and add the chicken thighs. Brown for 3-4 minutes on each side, until golden, then transfer to a plate. Add the onion to the pan and cook for 6-7 minutes until just softened, then add the red pepper and paprika and cook for a further 5 minutes over a low/medium heat.
Return the chicken to the pan and add the chopped tomatoes, tomato purée, bay leaf and oregano. Add a splash of water with some seasoning and cook with a lid on for 25-30 minutes, stirring occasionally to make sure the bottom isn’t sticking, until the chicken is very tender. Set aside to cool, then shred the chicken into small pieces. Return this to the rest of the filling mixture and stir through the olives. Discard the bay leaf.
Preheat the oven to 200°C/fan 180°C/gas mark 6. Take the pastry out of the fridge and roll out to a thickness of 3-4 mm. Using a 12cm round cutter, cut out 10 discs.
Divide the chicken mixture between them, leaving a roughly ½cm border. Brush the edges of the discs with water, then carefully fold over one half to make a parcel. Firmly press the edges together, then press with the end of a fork to give a crimped effect. Repeat with the remaining discs.
Put the empanadas on a large baking sheet lined with baking parchment. Place in the fridge for 30 minutes to firm up, then brush with the beaten egg and bake for 20-25 minutes until golden brown.