Peel and cube 800g potatoes. Add a pinch of turmeric and a pinch of salt to a pan of water, bring to the boil and boil the potatoes until almost cooked. Drain.
Heat 3 tbsp oil in a pan, add 1 tsp panch puran then one large chopped onion. Cook a few minutes then add 1 teaspoon each of minced garlic and ginger. Cook for 2 minutes, stirring, add a splash of water and then 1 tsp turmeric, 2 tsp coriander powder, 1 tsp cumin powder, 1 tsp garam masala, 1 tsp chilli powder, 1 tsp salt. Cook for a minute then add 1 tin of chopped tomatoes. Let this mixture simmer for 10 minutes, stir now and then.
Tip the potatoes into the pan and cook until they have absorbed all the flavours. Finish with chopped coriander, and sliced green chilli if desired.