Heat the oil in a large nonstick wok or frying pan, then add the mustard seeds. When they start to pop, add the garlic, chillies and curry leaves and fry for 1 minute. Add the onion and cook, stirring constantly, for 5–6 minutes.
Stir in the chilli powder, ground coriander, cumin seeds and turmeric and then stir in the tomatoes and sugar. Bring to the boil, reduce the heat and simmer for 8–10 minutes, stirring often.
Stir in the coconut milk and add the eggs and potatoes. Cook gently for 8–10 minutes until the sauce has thickened. Season with salt and serve immediately, garnished with chopped mint leaves.