The bread is what makes banh mi different from other sandwiches. Vietnamese bakers use rice flour as well as wheat flour, which gives the loaf a paper-thin crust that crackles when you bite it, and a fluffy, almost hollow inside. The bread is light enough that you can hold a whole sandwich in one hand.
Banh mi shops are a part of everyday life. Many are no more than a tiny stall on the pavement, with a basket of warm bread, jars of pickles, and a small grill. The seller slices the bread open, scoops in the fillings to order, and wraps it in paper - all in about a minute. Children buy them on the way to school. Workers grab one for lunch.
Inside, anything goes. Grilled pork, chicken, fried egg, tofu, sardines, meatballs - and always lots of fresh herbs, pickled vegetables and a hot chilli sauce if you want one. Each region of Vietnam, and each family, has its own favourite combination.
Banh mi has now spread around the world. In London, New York, Tokyo and Sydney, you'll find queues of people waiting outside little Vietnamese sandwich shops at lunchtime. It was named the world's best sandwich by lots of food writers.