Chop the onion and fry in 1 tsp olive oil until dark golden brown. Leave to cool on a plate.
In the same pan, heat another tsp olive oil. When hot, add the mustard seeds and let them cook until they start to pop. Add the curry leaves and cook for 10-15 seconds. Leave to cool for a few minutes.
Mix together yoghurt, onion, mustard seeds, and curry leaves in a bowl. Add salt and spices to taste. Serve at room temperature.