Choose a medium size and round shape onion. Cut it into two parts and carefully remove its petal.
For the masala, in a mixing bowl, add chopped onion, tomatoes, boiled and chopped potatoes, green chilli, salt, red chilli flakes, black pepper powder, cumin powder, chaat masala, lemon juice and some coriander leaves. Mix well. Masala is ready.
Then in a bowl, add besan, rice flour, rava, salt, turmeric powder, red chilli powder, garam masala and water. Make medium thick and ribbon-like consistency of the batter. Cover and rest for 10 minutes.
Now add onion cups into the batter, coat properly and remove the extra batter. (refer to video)
Heat oil and fry onion cups on medium flame, till they become crispy and golden brown from both sides. Make all cups in same way.
For assembling, take onion cups into plates. Then top it with masala, green chuntey, tamarind chutney and sev.
Serve crispy onion chaat immediately.