Add the onions, tomato, cucumber, green chillies, coriander leaves in a bowl. and combine.
Then add salt, lemon juice, cumin powder, chaat masala and mix well.
Notes
Chaat masala or chat masala is an Indian spice powder mix typically consisting of dried mango powder (amchoor), cumin, coriander, dried ginger, chilli powder, asafoetida (hing) black pepper and salt, easily available in most Indian grocery stores.