Soak the vadas in hot water for 10 minutes.
Drain out the excess water and set it aside.
Take the curd, water, sugar and salt in a mixing bowl and whisk it well to make it smooth.
Now take a small frying pan and add cooking oil to it. When the oil is warm, add mustard seeds.
Once the mustard seeds crackle, add curry leaves. Add this tempering in the curd and mix well.
Drop the vadas into the curd, mix gently and refrigerate for an hour.
Garnish with coriander leaves and boondi just before serving.