Chop the cucumber, ginger, green chilly, coriander leaves very finely. Grate raw mango finely. Keep it aside.
Now in a mixing bowl add drained split green gram, chopped cucumber, green chilly, ginger, coriander leaves, grated coconut and raw mango. Mix well.
In a seasoning pan add oil, mustard seeds, split black gram and curry leaves. When mustard seeds splutter add the seasoning to the kosambari.
Just before serving the kosambari, add salt and mix well. Cucumber kosambari is ready to serve.