Banh mi exists everywhere in Vietnam, but filling it with fish cake is how a coastal city makes one. The cake is fried to order so its edges crisp, then packed into the roll with cucumber, herbs, pickles and a stripe of chilli sauce.
The fish cake here is usually mackerel or lizardfish, minced and pounded until springy before it hits the oil — a different thing entirely from the pale, spongy industrial version. A good piece still tastes of fish rather than only of starch.
This is street food in the strict sense: bought from a cart, eaten while it is still hot. It costs very little and appears on most corners through the morning.