Cut cucumber vertically and remove seeds and chop finely. Transfer into a bowl, add salt and mix well.
Keep aside for some time. Place yogurt in a bowl, add salt and mix well. Squeeze cucumber well and add to yogurt and mix well.
Heat oil in a small non stick pan. Add mustard seeds, red chillies, curry leaves and asafoetida and saute till the seeds splutter.
Pour it over the pachidi. Keep it in the refrigerator for some time and serve cold.