Combine the onion, garlic and ginger in a food processor and chop together, but not to a paste, or chop with a knife and mix together.
Heat the oil in a karhai or heavy-based frying pan over medium heat and fry the onion mixture until softened. Stir in the turmeric. Add the potato and chilli and fry for 5 minutes. Add the chilli powder, cumin, coriander, salt and pepper and stir for ano