Add the flour, salt and butter to a large bowl. Using your fingertips (or a pastry cutter) rub the butter and flour until it resembles breadcrumbs. Drizzle in the water and mix using a fork until the it starts to come together and holds together when pressed in your hand. You may not need all of the water, you may need a little more.
Turn the dough out onto a floured surface. If the dough is too crumbly, add a little more water in tiny amounts. If too dry, do the same with flour. Roll the dough to a rectangle. Fold the edge furthest away from you to the center and fold the edge closest to you to the center so the edges meet. Turn 1/4 and roll out to a rectangle and fold the same way again. This step will make the dough light and flaky. Wrap the dough in plastic wrap and refrigerate for 30 minutes (for faster cooling, put in the freezer for up to 15 minutes).
Preheat oven to 425°F/220°C. Line a baking sheet with parchment paper.
While the pastry is chilling, add the sausage meat and sage to a mixing bowl and mix using your hands until well blended. Refrigerate until the dough is ready. Skip this step if not adding fresh sage.
Dust a clean surface with flour and roll out the chilled pastry into a rectangle, 24 inches (60 cm) long and 5 inches (13 cm) wide with the longest side of the dough closest to you with the rectangle going from left to right.
Shape the meat in a sausage shape down one side of the dough, about 1 inch (2/½ cm) thick, leaving a little pastry border. If the meat is sticking to your hands, rub a little oil on your hands. Brush beaten egg along the border (closest to the meat) then roll the other long edge to meet the egg washed egg over the meat, making sure to seal well and that the seam is on the bottom.
Cut into 8, 3-inch (7.5 cm) long rolls and place onto the baking sheet evenly apart. See note 2
Brush the rolls with the beaten egg and cut 2 slits into the top of the pastry taking care not to cut into the meat.
Bake for 25 minutes until they are golden and crispy.