IMO a good Bombay Sandwich has four things:
1) Vegetables that are cut just right - not too thick, not too thin. This to me is what makes it hard to find a good one.
2) Usually has three slices of bread to give it more structure. Not necessary but helpful.
3) A good green chutney spread.
4) A lot of shreaded cheese. In India they often use Amul cheese, a processed cheddar that is tangy and salty.
Good luck on your quest! 🤗