To make the pastry the fat needs to be rock-hard from the freezer, so first of all remove a pack of butter from the fridge, weigh out 75g then wrap it in a piece of foil and return it to the freezer or freezing compartment of the fridge for about 45 minutes.
Sift the flour and salt into a large, roomy bowl. Take the butter out of the freezer, remove the foil, or if you prefer fold back the foil and hold it in the foil, which will protect the butter from your warm hands. Then, using the coarse side of a grater placed in the bowl over the flour, grate the butter, dipping the edges of the butter onto the flour several times to make it easier to grate. What you will end up with is a large pile of grated butter sitting in the middle of the flour.
Now take a palette knife and start to distribute the gratings into the flour – don't use your hands yet, just keep trying to coat all the pieces of fat with flour. Now sprinkle a tablespoon of cold water all over and continue mixing with the palette knife, adding a further 1 to 2 tablespoons more water. Start to bring the whole thing together with the palette knife, and finish off using your hands. If you need a bit more moisture, that's fine, just dab on a little more with your fingers – just remember that the dough should come together in such a way that it leaves the bowl fairly clean, with no bits of loose butter or flour anywhere. Then pat the dough into a square shape and pop it into a polythene bag and chill for 30 minutes.
Meanwhile mix the sausagemeat, onion, sage and seasoning together in a mixing bowl, then divide it into two and roll each piece with your hands, on a lightly floured surface to 30cm long. Put them onto a piece of parchment, or a tray, whilst you roll the pastry.
Preheat the oven to 220°C, gas mark 7.
Now place the rested dough on a lightly floured surface and use a rolling pin that is absolutely straight. Lightly dust the pin with flour and rest it on the centre of the dough. Place the flat of your hands lightly on each end of the pin and begin to roll the dough backwards and forwards (don’t be tempted to roll from side to side), gently and evenly, re-dusting the pin and the surface very lightly with flour if you need to stop the pastry sticking. Then knock the sides gently with the rolling pin to keep an oblong shape and give the pastry quarter-turns as it expands. Roll the pastry to form an oblong approximately 30 x 20cm then cut it into two strips 30 x 10cm.
Place one roll of sausagemeat onto one strip of pastry. Brush the beaten egg along one edge, then fold the pastry over and seal it as carefully as possible. Roll the whole thing over so the sealed edge is underneath. Roll lightly and repeat with the second piece of pastry and sausagemeat. Then use a small sharp knife to cut each roll into six sausage rolls, each about 5cm long.
Snip three V shapes in the top of each roll with the end of some scissors and brush with beaten egg. Place the rolls on the baking sheet and bake high in the oven for 20-25 minutes, then remove them to a wire rack to cool.
You can store the cooked and cooled sausage rolls in an airtight tin, but they do lose their crunchiness. For this reason I think it is preferable to freeze them in polythene boxes and remove a few at a time as and when you need them. Defrost them for an hour at room temperature and then warm them in a hot oven for 5 minutes.