Banh mi means ‘bread’ in Vietnamese, but the term is also commonly used to refer to the uniquely Vietnamese style of sandwich that is sold all over Vietnam, and has become popular in Cambodia and Laos and more recently in Western countries particularly the United States of America.
There are many kinds of banh mi made in Vietnam and anyone who claims that a banh mi is only an authentic Vietnamese banh mi if it contains a certain list of ingredients has never been to Vietnam and is prone to food snobbery. For sure there are better and worse banh mi, but they are banh mi so long as they have the main defining characteristics.
About Banh mi
The most important characteristic that makes a sandwich a Vietnamese banh mi is the bread. Banh mi is made with a French style baguette with a difference.
The baguette was introduced to Vietnam by the French sometime in the middle of the 19th Century. It is difficult to put a precise date on the introduction of the baguette as French people went to live in Vietnam long before it became a French protectorate or any other identifiable historical event happened that we pinpoint as coinciding with the start of French influence on Vietnam. By the 20th Century the baguette had become part of the diet of Vietnamese people in large cities, but there was a problem with this popular dish: it cost a lot because it was made out of wheat which had to be imported. The Vietnamese found a way of getting around this problem by mixing wheat flour with rice flour thereby creating a different and uniquely Vietnamese style of baguette which is the most important defining characteristic of the Vietnamese banh mi sandwich.
Basically you can put anything in a Vietnamese style baguette and it becomes a banh mi, and indeed the Vietnamese put all kind of weird and wonderful filling in a baguette. For Westerners the strangest banh mi which is commonly eaten in Vietnam is the banh mi kep kem which is a baguette filled with ice cream and topped with crushed peanuts.
Traditionally, however, banh mi sandwiches contain one or more meat fillings and vegetables with flavouring such as soy sauce. The French traditionally like to eat baguettes filled with pate, ham or cheese. The Vietnamese have their own take on the fillings and often use local ingredients as either a replacement for the types of fillings which the French colonialists used or as an addition. This can mean many combinations from pork sausage with pickled vegetables to kebab meat with mayonnaise and salad.
The best known banh mi outside of Vietnam is the banh mi thit ngoui which is typically pate spread on the bread with beef or pork, herbs like coriander and raw vegetables, such as carrots cut into thin strips, with a spicy element, such as a chilli sauce, added. For many people who have only tried banh mi outside of Vietnam this is the dish that they associated with a authentic Vietnamese sandwich and for them anything is not a proper banh mi.