In a large frying pan add a drizzle of oil over medium heat and add potato. Fry until nice and crispy, then melt in your butter then add in onion. Fry off for a minute or two, just until soft. The longer you fry it the less powerful the onion flavor will be. For a stronger onion flavor keep the frying quick.
Pop everything in a bowl and allow to cool. To the bowl add cheese, mustard, parsley, cream/milk and a pinch of salt & pepper (to taste). Give it a good stir and leave to one side.
Slice the pastry into 8 equal rectangles and roll out into 6 inch squares.. Split your filling into 8 and place to one side of each rectangle. Brush around the edges with egg wash then fold over and push down with your fingers to secure. Using a fork, press down around the outside of each pasty. Place on a lightly sprayed oven tray and slice some steam holes in each pasty. (Holes are important to keep the pasty from getting soggy. Be decorative if you like)
Give the pasties a final brush of egg wash and place in the oven at 200C/390F for 15-20mins or until golden brown. At the 15 minute mark start watching the oven. Puff pastry can go from golden brown to burnt in no time at all.
