First make the pastry. Put the flour and butter into a bowl and rub together with your fingers until the mixture looks like fine breadcrumbs. Stir in the egg yolk and salt, then use a knife to gradually mix in the water until a dough forms. Bring it together with your hands, then wrap in cling film and chill.
Preheat the oven to 190ºC. Grease a shallow medium pie dish.
Heat a little oil in a frying pan. When hot, fry the onions and garlic until golden. Set aside.
Strip off the sausage skins and put the meat into a bowl. Add the onion mixture, the sage, parsley and mustard and mix together with a wooden spoon. Season well, then stir in half the beaten egg.
Lightly dust a work surface with flour and roll out the pastry to the thickness of a 50 pence coin. Use to line the prepared dish. Trim the edges and fill the pastry case with the sausage and onion mixture. Brush the rim of the pie dish with the second beaten egg.
Reroll the pastry trimmings to form a lid, place over the whole pie and trim off any excess. Using finger and thumb, pinch the pastry edges closed all the way around, like a pasty. Cut a little hole in the centre, then reroll the trimmings and cut out leaf shapes to decorate the top.
Brush the surface of the pie with the remaining beaten egg and bake for 30–40 minutes, until golden outside and bubbling inside. Serve with large dollops of Horseradish mash and a glass of cider.