I’ve actually never heard of Mumbai sandwiches but they seem to taste good so here’s how to make your own Mumbai sandwich:
To make the sandwich, toast slices white bread, and butter them on one side generously. Smear the bread with the Bombay sandwich spread, or a combo of coriander and green chile chutney (or any other chutneys you have around). Add generous amounts of raw diced white onion, slices of tomato, sprigs of cilantro and slices of boiled potato (optional). Sprinkle on some chaat masala if you have it.