Boil the diced potatoes in salted water for 6–8 minutes or until just tender. Drain and set aside.
Heat oil in a pan. Add cumin seeds and cook for 1–2 minutes until fragrant.
Add onion, ginger, chilli and a pinch of salt; cook 5 minutes until softened.
Stir in coriander, cumin, garam masala and cayenne; cook 1 minute. Add peas and cook 2–3 minutes.
Remove from heat, add potatoes, lemon juice and coriander. Season to taste. Cool completely.