Wash the cucumber and grate into a bowl. Set the grated cucumber in a strainer and set aside to drip. This will help remove the excess water in the cucumber.
In a small pan, dry roast the cumin seeds until fragrant and slightly browned. Let cool a little, then use a mortar and pestle to crush the seeds.
Finely chop the onion and enough cilantro to measure 1⁄4 cup.
Add all the ingredients to the yogurt and season with salt and freshly ground pepper.