Heat the oil in a large nonstick wok or frying pan over a medium-high heat. Add the mustard seeds, chilli powder, cumin seeds and curry leaves. Stir-fry for 1–2 minutes until fragrant.
Add the ground spices and potatoes. Season to taste and stir-fry briskly over a high heat for 4–5 minutes. Remove from the heat and stir in the coriander. Squeeze over the lemon juice just before serving.