Heat the oil in a wok over a medium heat.
Add the cumin seeds and mustard seeds and stir-fry for 1–2 minutes. Add the onion, ginger and chillies and cook for 3–4 minutes.
Add the cooked potatoes and peas, and stir-fry for 3–4 minutes. Season well and stir in the lemon juice and coriander leaves. Divide the mixture into 25 portions and shape each one into a ball. Chill until ready to use.
Make the batter by mixing together the gram flour, self-raising flour and rice flour in a bowl. Season and add the turmeric and coriander seeds. Gradually whisk in the measured water to make a smooth and thick batter.
Pour sunflower oil into a wok until one-third full and heat to 180–190°C (350–375°F), or until a cube of bread browns in 30 seconds. Dip the potato balls in the batter and deep-fry in batches for 1–2 minutes or until golden. Drain on kitchen paper and serve warm with mint chutney.