Heat oven to 200c. Roll out the pastry to make a large square. Cut it into 8 squares. Mix together the mash,cheese,onion & milk & place a good sized spoonful into the centre of each pastry square. Moisten the edges of it & fold over,using a fork to seal the edges.
You can freeze them from this point.
Put onto a baking tray & cook for 20mins.
When I make cottage pie I make extra mince mix and use that to fill some - and the same for chilli - but the cheese and onion one is the I do the most