Peel and grate the cucumber. It will ooze out water and don't discard the water cucumber water;
In a mixing bowl, combine the grated cucumber, yogurt, and salt. Mix well.
Grind the coconut and green chili with ¼ to ⅓ cup of water (you can also use the cucumber water here) into a smooth or slightly coarse paste. Add this to the cucumber-yogurt mixture and stir to combine.
Heat oil in a small tadka pan. Once hot, add the mustard seeds, urad dal, and chana dal. When they start to splutter, add the asafoetida and curry leaves and let them sizzle for a few seconds.
Pour the tempering over the raita and mix well. Taste and adjust salt.
Garnish with cilantro and serve immediately or chill for 15–20 minutes before serving.
Notes
Always taste a small piece of cucumber before grating — a bitter cucumber will ruin the raita.
Don't discard the water that releases when grating the cucumber — it has all the goodness. You can use it while grinding the coconut paste, though I didn't use it in this recipe; I added the coconut directly to the yogurt and used water for grinding the coconut masala.
Add the tempering just before serving if you're making this ahead.