Make the Pastry
Put the flour and salt in a large mixing bowl. Rub in the cubed butter between your forefingers until the mixture resembles coarse breadcrumbs.
In a separate measuring jug, whisk together the egg, milk, and vinegar.
Add the wet ingredients into the dry. Using your hands, bring the mixture together until it forms a dough – it should be soft but not sticky. Shape the dough into a ball on a flour-dusted work surface, flatten into a disc and cover in cling film. Set aside in the fridge for at least 60 minutes to rest. The pastry can be made up to 24 hours in advance. It also can be frozen for up to 8 weeks.
Make the Potato and Cheddar Filling
Melt the butter in a heavy-bottomed pan over medium low heat. Add the onions and saute for 5 minutes until softened.
Add the potato and sage to the onions and continue to cook for 5 to 7 minutes or until the potatoes are tender. Remove from the heat and set aside to cool in a bowl for 20 minutes.
Stir the cheddar and cream through the cooled potato mixture, season with a good pinch of salt and lots of freshly ground black pepper.
Assembling the Pastries
Preheat the oven to 375ºF (190ºC). Divide the pastry into 6 equal pieces. Roll out the dough on a lightly floured surface to form roughly a 6 ½ inch circle. Pile about ⅓ cup of the cooled filling just off center on the pastry, leaving a border around the edge.
Fold the other half over and press to enclose the filling. Crimp the edges of the pastry and place on a large, lined baking sheet. Repeat with the remaining pastry and filling.
Generously brush the pastries with the egg wash and bake for 25 to 30 minutes, until golden.
Serve the pastries, hot from the oven with salad on the side. Store leftovers, covered in the fridge for up to 2 days.