Heat the oil in a small saucepan or kadai
Add the mustard seeds to the oil and cover.
When the seeds are done popping, remove the lid, and turn down the heat.
Add the urad dal and green chilis and give them a quick stir.
Add the diced onions and cook while stirring for a few minutes until they begin to turn translucent. We don’t need to cook the onions fully, just until they lose their raw crunch. Be careful not to cook them on a high heat or they will turn the raita a brown color if they caramelize.
Add the tomatoes and stir for a quick 30 seconds. We are not trying to cook the tomatoes, just heating them enough to help them release their juices in the raita.
Add most of the cilantro, reserving a little bit for garnish
Turn the heat off, the cilantro will wilt in the residual heat.
Add the vaggarnae and salt to the prepared yogurt and mix everything together.
Taste the raita and adjust the salt at this stage. If the raita seems to be too thick, you can dilute it with a little water. Remember that the onions and tomatoes will also release some juice.
Garnish with the fresh grated coconut and the rest of the cilantro.
Chill the raita in the refrigerator until ready to serve.