In a bowl, whip the yogurt until it is creamy.
Fold in the onion, tomato, green chilli, and salt.
Heat the oil in a small frying pan, then add the mustard seeds.
Fry for a few seconds until you can smell the aroma, then tip in the peanuts, if using.
Fry for a minute, making sure they don't burn.
When the mixture has cooled gently add it to the yogurt.