Heat oil in a pan and add mustard seeds, when they crackle add urad dal and curry leaves. Add the chopped onions and green chilli and cook for a minute or so. Next add grated radish and keep stirring for a couple of minutes until raw smell disappears.
Remove from the flame and let it cool completely.
Beat or whisk the yogurt with salt and add cooked radish to it.
Adjust yoghurt, until you get the desired consistency.