Canh chua, the sour soup at the centre of the table

Canh chua is the bowl in the middle of a southern table. Tamarind-sour, full of pineapple and fish, and meant to be spooned over rice rather than eaten on its own.

What is actually in it?

A tamarind broth, fish, and a specific set of vegetables that almost never varies. Pineapple in wedges, tomato, okra, bean sprouts and bac ha, the spongy diagonal-cut stem that soaks up broth and keeps its crunch.

Rice paddy herb, sawtooth coriander and fried garlic go on top, off the heat. Snakehead is the classic fish, prawns are common and catfish is delta standard. Tofu makes the version a vegetarian table can eat.

Why is it not a noodle soup?

Because it is not a dish for one person. Canh, in Vietnamese, is the shared soup that sits among the other plates. Everyone ladles a little of it over their own rice through the meal.

The difference from pho or bun bo Hue is exactly that: those arrive in a bowl with your name on them. Ordering canh chua alone in a restaurant is possible and slightly odd, like ordering gravy.

What is it served with?

Ca kho to — fish braised dark in a clay pot with caramel and pepper — is the pairing a southern kitchen reaches for automatically. Sour and salty, wet and dry, and a plate of rice between them.

Thit kho does the same job when the braise is pork. The logic is always the same: one sharp dish, one deep one, and rice underneath both.

Is the sourness the same as tom yum?

The shape is similar and the seasoning is not. Both are sour-hot soups with fish or prawns, and both come from a kitchen that thinks about balance rather than about courses.

Tom yum takes its sourness from lime, and galangal, kaffir lime leaf and chilli carry the aroma. Canh chua takes sourness from tamarind and sweetness from pineapple. The aroma is rice paddy herb, a smell nothing else in the region uses this way. The heat is optional and the table adds it.

How long does it take to cook?

Twenty minutes, which is the other reason it is on every table. Tamarind pulp is soaked and strained, the broth comes to a boil, the fish goes in for five or six minutes and the vegetables go in by hardness, pineapple first and bean sprouts last.

Everything aromatic is added off the heat. Rice paddy herb, sawtooth coriander and fried garlic go on at the end, because a minute of boiling takes the smell out of all three. A pot serves four and costs a household perhaps 60,000 dong of ingredients.

Which part of the delta does it best?

The Mekong Delta, where the fish is an hour old and the tamarind is local. Can Tho, Ben Tre and the rice shops of the smaller towns all serve it as a matter of course.

In Ho Chi Minh City any com binh dan has it on at lunch. A ladle over a rice plate costs a few thousand dong. Restaurants aimed at visitors do it badly. The dish is built to be shared and they serve it as a single portion.

What does a bowl of canh chua cost?

Almost nothing as part of a rice meal: a plate of rice, a braise and a ladle of canh chua runs 35,000 to 60,000 dong at a counter. Ordered as a full bowl for a table in a restaurant it is 90,000 to 200,000 dong depending on the fish.

At about 26,000 dong to the dollar that is a meal for two dollars and a shared soup for four to eight. The cheap eating guide covers where those numbers come from.

What should you look out for?

Bones. Cooks cut snakehead and catfish across the body, bone in. Diners eat them off the bone rather than filleted, which is normal here and catches visitors who expect a fillet.

The other thing is heat: the soup is served hot but not spicy, and the chilli is on the table. Adding it changes the dish more than it looks like it should, so try a spoonful plain first.