Grate the potatoes working them on the grater in one direction only.
Put them in a bowl, add salt, roasted cumin seeds, red chillies, pepper powder, spring onions and coriander leaves and mix with a lit hand, incorporating air as you mix.
Mix a non stick pan on medium heat. Add one tablespoon butter and spread it around the pan.
Spread a quarter of the potato mixture in a thick layer. Put half a tablespoon of butter over the potatoes and cook on low heat till the underside is golden and crisp.
To flip, place a plate over the roesti and turn the pan upside down. Then gently slide the roesti onto the pan once again so that the other side can be cooked to a crisp golden brown. Serve hot.