Peel, halve and deseed the cucumber. Chop into a fine dice.
In a pan, add the diced cucumber with a splash of water (less than 1/4 cup), salt and finely chopped green chilli. Cover and cook until cucumber is soft and water has evaporated.
Meanwhile, grind the coconut, cumin, mustard seeds to a paste with 1-2 tbsp of water.
Add this paste to the cooked cucumber.
Remove into a bowl and cool for 10 minutes.
Whisk the yogurt well and mix it into the bowl with the cucumber. Add a pinch of salt if required, as per taste.
Heat oil in the pan. Add the ingredients for tempering and transfer over the kichadi.