Sift the flour and salt into a large bowl and add the butter. Mix the butter around with a large metal spoon to coat it in flour. Be careful to keep the butter in lumps.
Mix the water and lemon juice together in a jug and gradually pour it into the flour and butter mixture.
Using a round-tipped knife, cut across the contents of the bowl several times, turning the bowl continuously as you chop the butter into the flour, until the dough comes together. The dough is very wet at this point.
Tip it onto a lightly floured work surface and quickly shape it into a rectangle about 30cmx20cm/12inx8in. With the pastry vertically on the board, fold the bottom third of the pastry up onto the middle third, then the top third down onto the other thirds. Wrap in cling film and chill for 10 minutes.
Bring the pastry out of the fridge and with the folded edge to the sides, roll the pastry again into the same proportions as the original narrow rectangle and fold in the same way again. Chill again. Repeat this twice more. After the last folding stage, wrap the pastry in cling film and chill in the fridge for two hours.
For the onion chutney, place the onions, caster sugar and balsamic vinegar into a medium saucepan. Cook gently, over a low heat, stirring occasionally, for 15 – 20 minutes or until the onions are soft, translucent and lightly caramelised.
Place the sausagemeat, chopped thyme, salt and freshly ground black pepper into a large bowl and mix thoroughly.
When the pastry is ready to use, if it is very firm, allow it to warm in the room for a few minutes.
Pre-heat oven to 220C/425F/Gas 7. Cover a large baking sheet with baking parchment.
Place the pastry onto a lightly floured board and roll out to a rectangle about 36cmx30cm/15inx12in. Cut into 6 even rectangles measuring about 12cmx15cm/5inx6in.
On the short side of each small rectangle place a cylinder of seasoned sausage meat weighing about 100g/3½oz. Spread some onion chutney onto the remaining pastry.
Wet the short edge of the pastry with a little water and roll up the sausagemeat in the pastry. Place onto the lined baking sheet with the seam underneath. Repeat with the remaining sausagemeat and pastry.
Glaze each sausage roll with the beaten egg. Diagonally slash each sausage roll on the top seven times.
Cook for 30 - 35 minutes or until the pastry is golden-brown and the sausage is cooked through. Eat warm or cold.