Pre-heat the oven to 200°C. Grease an oven dish.
Chop the onion.
Grate as much cheese as you like.
Place the mashed potato in a large bowl, add in the onion and cheese (leave some cheese to sprinkle over the top).
Sprinkle remaining cheese and bake in the oven for around 30, just until the top is golden and the cheese has melted. Serve immediately with some nice meat or eat with soft bread. Enjoy!