Mix dahi with onion slices, tomatoes and cucumbers in a bowl. Put the bowl in the fridge for 10 minutes.
Heat a pan with ghee, and add cumin seeds, curry and coriander leaves. to it. Fry until the seeds turn a bit brown. Take the pan off the heat and allow the ingredients to cool. Pour this mixture on to the chilled mixture of tomatoes and cucumber. Mix.
Sprinkle black salt and sugar to the mixture. Stir and mix well. Chill in fridge for 10 minutes, and serve.