Pav bhaji is made on a large flat iron griddle rather than in a pot, and the equipment is not incidental. The method depends on the surface being wide, hot and continuously in use.
Mashing needs a surface to press against
The vegetables are broken down against the griddle with a heavy masher, and a flat plate gives a firm resistant surface across a wide area for that motion.
A pot's curved walls concentrate the mixture in a small area and make effective mashing awkward. The texture that results is chunkier and more stew-like than the dish intends.
Continual mashing on a hot surface also means the mixture is being cooked and worked at the same time, which is how it reaches a uniform consistency without turning into a puree.
Reduction requires exposed surface
Boiled vegetables carry a lot of water, and the dish needs to lose most of it to reach a spreadable rather than a soupy consistency.
Spreading the mixture thin across a large griddle exposes an enormous surface to the air, so evaporation is fast even at moderate heat. A pot would need much longer and would risk scorching the base.
The same surface toasts the bread
The rolls are split, buttered and pressed onto a clear section of the same griddle, picking up color and some of the residue from the bhaji cooking nearby.
Having both operations on one surface removes any need for a second heat source, which matters for a cart with limited fuel and space.
It also synchronizes them, since bread and bhaji finish together and the plate goes out immediately rather than waiting on either component.
Volume production is built into the design
A vendor keeps a large mass of partly finished bhaji on the cooler end of the griddle and pulls portions forward to finish with butter and seasoning to order.
That two-zone approach means each plate is finished individually while the bulk cooking happened once. It is why one griddle can serve a queue continuously.
Reproducing it in an American kitchen
A large cast iron griddle spanning two burners is the closest domestic equivalent, and it works for both the mashing and the bread. A wide heavy skillet is a smaller-scale substitute.
Soft dinner rolls or slider buns are the practical US stand-in for pav, since what the dish needs is a soft, square, slightly sweet roll that can be split and pressed flat.