The onion bhaji sandwich is a sandwich whose entire success rests on keeping a fried shell crisp inside bread that wants to steam it soft. A bhaji is sliced onion bound in a spiced gram-flour batter and deep-fried into a craggy, brittle fritter, and the moment it goes between two slices it starts fighting its own enclosure: trapped steam from the hot fritter has nowhere to go, condenses against the bread, and turns the crisp lattice back into something damp and heavy. The defining craft is delaying that. The bhaji has to be drained hard and as freshly fried as the build allows, and the bread chosen and dressed so it does not accelerate the sogging from its side.
The craft is moisture management on both faces. A soft white bap or roll is split and the cut faces are sealed, with butter or a thicker layer of a yoghurt or mint sauce, so the bread is waterproofed against the fritter's oil and steam rather than soaking it up. The sauce is doing structural work as much as flavour work: a cool, sharp raita or a mint-and-chilli chutney is what makes an intensely spiced, oily fritter edible in one hand, and it is applied as a measured layer, not a flood, because too much liquid drowns the very crunch the sandwich exists for. The bhaji is pressed only lightly, since crushing it collapses the open structure that holds the crispness. A salad leaf or sliced raw onion adds a fresh, water-crisp counter against the fried density.
The variations track the takeaway counter. The roll version uses a soft floured bap with chutney and salad; the wrap folds the same fritter into a warmed flatbread with raita, trading the crisp-bread problem for a fold that holds a looser load. A double-bhaji build leans heavier and needs more cooling sauce to stay balanced. The pakora roll is the close gram-flour relative built on a different fritter, and the samosa sandwich pushes another fried snack through the same bread-and-chutney logic. Each deserves its own article rather than being crowded in here.