Put the flour, sugar and salt in the bowl of a food processor fitted with a metal blade. If you have time place in the freezer to chill for 10 minutes.
Add the cubed butter and shortening and pulse briefly until the mixture looks like breadcrumbs. Don’t overdo it though – you still need some pieces of butter the size of small peas.
Mix the water and vinegar and add it very gradually through the feeding tube while pulsing in short bursts until the dough forms clumps – you may not need to use all the water.
Place a large piece of clingfilm on your worktop and tip the dough on it. Pat the dough into a rectangle shape, wrap and chill for at least an hour or up to a day.
Dust your worktop with a little flour and roll the dough out to a long rectangle. Take one third of the pastry and fold towards the middle and fold the remaining third over the top to make three layers.
Repeat the rolling out and folding once more until the dough is elastic. Fold into thirds again, wrap in cling film and chill for another hour (or overnight) before using.
Preheat the oven to 220C (200 Fan). Line two trays with baking paper.
Carefully remove the skin from the sausages and set aside.
Cut the pastry in three pieces, keeping the pieces you are not using covered and in the fridge – especially if the day is hot!
Roll the pastry out to a rectangle 15x30cm and roughly the thickness of a pound coin. Place a scant amount of the onion chutney in the centre of the pastry. Position two sausages over the chutney and cut the rectangle in half.
To create the sausage plaits, cut strips on the diagonal either side of the filling. Moisten the edges with a little egg wash. Fold the strips alternately towards the middle to create the plait.
Lightly brush with egg wash and sprinkle with the nigella seeds. Place on baking tray and repeat with remaining pastry.
Reduce the oven temperature to 200C (180 Fan) and bake the plaits, in batches if necessary, for 25-30 minutes or until the pastry is golden.
Serve with a little mustard and ketchup as dipping sauces if you like.
To create sausage tolls, follow the method up to step 10. Moisten the left edge of the pastry with a little egg wash. Starting from the right, fold the pastry over the sausage and then over again to encase the fillings. Press the edge to seal.
Put in the freezer to chill for 20 minutes and then carefully cut into smaller rolls. Brush with the egg white and sprinkle with seeds. Place on a tray lined with baking paper and bake for 15-20 minutes.