Heat the oil in a large saucepan until hot, add the mustard seeds, onion, curry leaves and chilli flakes and fry, stirring, for 4–5 minutes or until the onion is lightly browned.
Add the ginger and chilli and stir-fry for 1 minute, then add the turmeric and stir to mix well.
Remove the pan from the heat, add the mangoes and yogurt and stir continuously until well mixed. Season to taste with salt.
Return the pan to a low heat and heat through for 1 minute, stirring continuously. (Do not let it boil or the curry will curdle.) Serve immediately with 4 warm chapattis.