Pre-heat the oven to 200°C (400℉).
Grease an oven dish (the size you need will depend on how much cheese and potato pie you have).
Chop the onion.
Grate as much cheese as you like.
Place the mashed potato in a large bowl, add in the onion and cheese, and anything other ingredients you might want to use.
Give it a good stir, just until all ingredients are evenly distributed.
Sprinkle more cheese over the top.
Bake in the oven for around 30 minutes, just until the cheese has melted and the top starts has browned.
Serve immediately as a side, or by itself with some soft bread.