Spiced yoghurt

– 255ml/9fl oz natural unsweetened yoghurt
– 1 small Spanish onion, finely sliced
– 1 medium tomato, finely chopped
– 1 green finger chilli, finely chopped
– ¼ tsp salt
– 1 tbsp vegetable oil
– ¼ tsp brown or black mustard seeds
– 4 curry leaves
– 10 unsalted skinned peanuts (optional)