Cook the chopped cucumber with salt, adding little water till it turn to soft.
Grind the grated coconut, green chillies, ginger and 1/2 tsp mustard seeds into a smooth paste with 1/4 cup yogurt.
Add the coconut paste to the cooked cucumber and cook over low flame. Allow to cool and add the rest of the yogurt and set it aside.
Heat cooking oil for tempering in a small pan and add the curry leaves, mustard seeds, when the mustard seeds start popping, add it to the pachadi and mix well.
Serve with steamed rice and enjoy!