In a blender or mixer, grind shredded coconut, green chilies and salt to get a coarse consistency. To this add curd, mix well and keep aside.
Heat oil in a pan or kadai, add mustard seeds. Once the mustard seeds stop splattering add split black gram, curry leaves. Once the curry leaves stop spattering add the chopped onion and sauté until they become translucent (golden brown).
Now add this to the ground chutney, mix well and serve.