The Bombay sandwich is a vegetable sandwich that behaves like a spiced one, and the trick is a layer most people never see: a thin film of green chutney painted onto the bread before anything else goes on. Underneath the cucumber, tomato, boiled potato, and cheese is a coriander and mint paste sharpened with green chilli and lime, and it is doing the structural work that mayonnaise does in a plainer sandwich. It seasons every bite from the bread up rather than from a clump in the middle, and it waterproofs the crumb against vegetables that are mostly water. Without that chutney the build is a stack of bland slices. With it, the same slices read as deliberately spiced. That painted layer, not the filling, is what defines the sandwich.
The craft is moisture management against a wet load, then heat applied to set it. Tomato and cucumber weep, and boiled potato is starchy and damp, so the chutney film on both inner faces of the bread is the barrier that keeps the slices from soaking the crumb to paste before it is eaten. The vegetables are cut thin and even so the sandwich presses flat and each bite carries the same ratio rather than a slab of potato at one end. A dusting of chaat masala, a sour, slightly sulphurous spice blend, goes over the layers to lift them past plain salt. The closed sandwich is then buttered on the outside and pressed in a hot iron until the bread crisps and the cheese inside slackens, which welds the loose vegetable stack into something that holds when cut. The grilling is not optional finish; it is the step that turns a fragile salad sandwich into one that survives being handed across a counter and eaten standing up.
The variations sit inside the same chutney-and-press logic. The ungrilled version is served cold and cut into fingers, lighter and more fragile. Adding boiled beetroot bleeds colour and earthy sweetness through the build; swapping in a fried potato patty pushes it toward a heavier street sandwich; the grilled cheese-forward version leans on melted cheese as the binder rather than the vegetables. Each deserves its own article rather than being crowded in here.