Wash cucumbers, onion (if using) and green chilies under running water. Peel the cucumbers if you prefer and chop them to bite sized pieces. Deseed chilies if you prefer low heat and chop them. Fine chop coriander leaves and onions.
Add yogurt and salt to a mixing bowl and whisk well until smooth. Add cucumbers, onions, coriander leaves, chilies, salt and cumin powder. Mix well and taste test to adjust salt. If it is too thick for your liking add more yogurt.