Grind coconut, red chilli, mustard and cumin to a fine paste using little water.
Chop the tomato and put it in a kadai. Add a tablespoon of water, turmeric powder and cook it on medium flame just for one or two minutes or it turn soft.
Add the coconut paste and mix well. Allow to boil. Remove from stove and cool it.
Add sugar and salt to the curd and beat it well. Add the tomato mixture to the beaten curd and mix it well.
In a small kadai put the oil and when it is hot add mustard. When it pops up, add asafotida powder, red chillies and curry leaves. Pour this seasoning over the curd. Close with a lid and refrigerate it for atleast half-an-hour before serving.