Trim the sides of the bread slices, apply butter and coriander and mint chutney. On a flat board, arrange four bread slices with chutney spread, layer it with onion, cucumber, tomato roundels and potato slices.
Sprinkle some chaat masala, salt and crushed peppercorns. Cover each of them with another slice of bread and press lightly.
Cut each sandwich into six equal portions and serve it with tomato ketchup.