Cheese toast and chilli cheese toast are pretty ubiquitous in Mumbai, at home and outside (but not street food, usually — the “Bombay sandwich” — which is a mixed vegetable sandwich — is street food).
They can be open-faced or closed (cooked in what looks like a pie iron but is a stovetop sandwich toaster).
What makes it a chilli cheese toast in its basic version is just chopped green chillies on top of (open) / mixed into (closed) the cheese. Beyond that, vegetable add-ins (finely minced red onion, tomato, capsicum, cilantro, even corn), green chutney, red chutney, Szechuan sauce, red chili flakes, even oregano these days.
The bread (closed) / toast (open) is always buttered before adding cheese. No mustard. Accompanied by ketchup always, but also possibly green chutney or Szechuan chutney or whatever else strikes someone’s fancy.
Highly recommend. I want one now, but I’m out 
I don’t know what “traditional” means in this context @Harters — what’s the date cut-off? Pre-colonialism? But then all “pao” / bread items would go out of the window, because they came with the Portuguese
(Re Eggs Kejriwal — the original is a fried egg on buttered toast, doused in green chutney and cheese so that the egg isn’t visible. Created for a vegetarian member of the club (Mr Kejriwal) who did not want to be outed for eating eggs — so hiding the egg under something was the whole point )
