Bun mam, the delta bowl built on fermented fish

Bun mam is the bowl the Mekong Delta is proudest of and the one it warns visitors about. The broth is built on fermented fish, loaded with seafood, and unlike anything else in Vietnamese cooking.

What is actually in the broth?

The broth starts with mam, whole small fish salted and fermented for months until they break down. Cooks simmer the mam with water and strain out the bones. The liquid is then built up with lemongrass, sugar and stock until it is savoury rather than merely salty.

The straining is the whole trick, and it is why the bowl divides people. Done badly it is a salt bath. Done well it has the depth a long-cooked anchovy sauce gives a European dish, with the sweetness of the prawns sitting against it.

What comes on top?

A full bowl carries prawn, squid, a slice or two of pork belly, roast pork and fish. Rounds of aubergine cook in the broth until they collapse. The noodles are thick round rice noodles rather than the flat ones in pho.

The herb plate is not a garnish. Water lily stems, banana blossom, bean sprouts, rau ram and perilla all go in by hand as you eat. The bowl is designed around that cutting of the broth, and the herbs guide covers which is which.

Where does the delta rate it best?

Soc Trang and Chau Doc are the two places the delta itself names, and they make it differently. Soc Trang's version is the Khmer-influenced one, sold from early morning in the market. Chau Doc leans on the mam its own river makes.

Can Tho sells the most of it, because it is the biggest city in the region. In Ho Chi Minh City the good versions are in the western districts, where the delta families settled, rather than in District 1.

How long does the mam take to make?

Months, and that is the number that explains the price and the smell. Small fish, usually linh or sac, are packed with salt in jars and left for three to six months, sometimes a year, until the flesh has broken down into a brown paste.

A kilo of finished mam makes a great deal of broth, which is why a bowl still costs 30,000 to 60,000 dong despite the work behind it. Markets in Chau Doc, 250 km from Saigon, sell it by weight from open basins, and the mam section is the part of the market visitors remember.

Why does it smell like that?

Because fermentation is the point, not a side effect. Mam is the delta's method of keeping a glut of river fish through the year. The same technique stands behind the fish sauce on every table in the country, and the fish sauce guide explains how one becomes the other.

A pot of bun mam announces itself from across a market, which is the honest warning. It is also why a stall selling it usually sells nothing else.

When do you eat it?

Breakfast and lunch, like most Vietnamese soups, and it is heavier than the bowls a visitor usually meets at that hour. Stalls open around 6 and sell out by early afternoon, and a bowl runs 30,000 to 60,000 dong. A counter still ladling at 4 has been sitting on that broth for a while.

The breakfast guide covers what else appears at that time of day, and the noodle soups guide places this bowl among the rest of the family.

What should you order the first time?

Order a small bowl at a busy counter and ask for it without the strongest toppings if the smell worries you. Many stalls will build a version heavier on prawn and lighter on fish, which is a gentler entry to the same broth.

Eat it with the herbs rather than before them, squeeze the lime, and use the chilli. A bowl of bun mam taken plain is a test of endurance. Taken the way the delta eats it, it is the most interesting thing on a southern menu.