For the pastry: Put the flour and salt into a large bowl. Add the cubed butter and using a blunt knife roughly mix the butter into the flour. You want to maintain lumps of butter, so don’t mix too hard. Pour in half of the water and stir to combine, then pour in the remaining water until the mixture just comes together (you may not need all the water).
Tip the mixture out onto a floured work surface and roll into a long rectangle with the short edge facing you. You want it to be around 15cmx45cm. Then fold the top third of the dough down over the middle third, and fold the bottom third up over the two-thirds, as if you are folding a letter. Turn the dough 90 degrees so the open ends are facing you and repeat the rolling and folding process. Place in the fridge for 30 minutes, then repeat the rolling and folding process twice more, before chilling again for 1hour.
Preheat the oven to 200C/180C Fan and place baking paper over a large baking tray.
For the filling: Firstly slice the red onions finely and place in a large saucepan over a medium-low heat with a little oil. Cook, stirring every now and then until they are well softened and about to caramelise. Then take off the heat and leave to cool slightly. In a large bowl add the sausagemeat, egg, breadcrumbs and parsley and combine thoroughly – you may need to use your hands. Then season well.
To assemble, roll the chilled pastry out into a large square – approx. 40cm. Then divide into three rectangles, with the short ends facing you. Smear English mustard down the middle of each rectangle, and top with a squiggle of ketchup and a layer of onions. Divide the sausagemeat filling into three long sausages and place on top of the onions.
Using a knife, slice the pastry diagonally down each side of the filling (see picture above) and then wrap over the filling alternating each side to create a plait. Cut each long sausage into 4 so you end up with 12 sausage rolls. Place them onto the baking tray and brush the beaten egg over each sausage roll. Bake for 25-28mins until golden brown. Leave to cool a little, then enjoy.
Sausage rolls can be kept in an air-tight container in the fridge for up to 4 days.