1Grind most of the coriander, mint, one green chilli, lemon juice and salt with a splash of water into green chutney.
2Heat oil, crackle mustard seeds and curry leaves, add chopped onion, remaining chilli and turmeric, then the potato and salt. Mix well and cool.
3Butter the bread, spread chutney, then a layer of potato. Top with tomato, cucumber and onion slices, chaat masala and cheese. Close with another slice.
4Toast in a sandwich griller or on a tawa, buttering the outside, until deep golden and crisp.
5Cut into quarters and serve with ketchup and more chutney.
Serve it like this
Stack them in a cloth lined basket so they stay soft and warm, with a little ghee on top if you like.