In a pan, add the ragi flour and water. cook until it becomes thick, without any lumps. keep it aside.
Take a blender jar, blend cooked ragi and buttermilk together.
For garnishing, In a separate small pan, heat oil, add mustard seeds and allow it to splutter.
Add Urad dal, Channa dal into them and cook until Channa dal becomes golden brown.
Add it to the ragi buttermilk and serve chilled or at room temperature.