@mike There are lots of recipes online - it is a favourite street food. Each recipe has it's own twist.

This one looks good but add cooked beetroot to it. Australian tinned beetroot would work well, or some supermarkets sell vacuum packed cooked beets, if you don't want to cook your own. I don't add cheese, but feel free. https://www.honeywhatscooking.com/bombay-grilled-sandwich/

You can buy chaat masala and coriander chutney in any Indian grocery.

I'll also send you a recipe that I have that is approx similar and also relatively easy to make.

Indian sandwiches can take as much work as a meal by the time you grind your spice mixes and make your chutney spread(s) 😆