Roll out your pastry until its a big oblong
Cut into approximately 4 large squares
Make up smash potato and add chopped onion and cheese.
Pile into centre of each square
Brush the edges with milk and fold over to make into triangles.
Cook in oven at temperature for cooking puff pastry (220ish) for about 20 mins or so, until all puffed up and golden brown.....these are seriously delicious