Bun bo Hue, the bowl the old capital eats at breakfast

Hue served as Vietnam's imperial capital from 1802 to 1945, and it serves its most famous dish before most visitors are awake. Bun bo Hue is a breakfast soup in the city that invented it. Pavement kitchens start selling at six in the morning, and the good ones have sold out by the time a lunch menu appears elsewhere.

What is actually in a bowl of bun bo Hue?

A bowl of bun bo Hue holds thick round rice noodles, sliced beef shank, a length of pork hock and a red broth. The lemongrass in it announces itself before the bowl reaches the table. A disc of cha, the mild steamed pork sausage, usually sits on top. Cubes of set pork blood sit under the surface unless you asked for them to be left out.

The noodles are the first thing to notice. Bun is round rice vermicelli rather than the flat sheets cut for pho. The strands here are the thickest of that family, closer to spaghetti than to a ribbon, and heavy enough to survive a broth this strong.

Why is the broth red rather than clear?

Chilli oil gives bun bo Hue its colour, and lemongrass and fermented shrimp paste give it everything else. The oil is usually a satay paste of chilli, lemongrass and annatto. It goes into the pot rather than onto the bowl at the end, so the heat runs through the whole soup instead of sitting on top of it.

Mam ruoc is the ingredient a visitor cannot place. It is the fermented shrimp paste of the central coast, and a spoonful of it in a stockpot is the difference between a beef soup and this one. It smells assertive in the jar and disappears into savouriness in the broth.

The chilli oil and the shrimp paste belong to no bowl of pho, which is the point. The two soups come from opposite ends of the country and from opposite ideas about what a broth should do.

How hot is bun bo Hue, and can you turn it down?

Bun bo Hue is the hottest of the big Vietnamese soups, and the heat is built in rather than added at the table. A bowl ordered without warning in Hue will be hotter than the same dish sold in Hanoi, where cooks have quietly adjusted it for local taste.

Khong cay means not spicy, and it is worth saying before the bowl is assembled rather than after. Most stalls will then leave the chilli jar and the satay spoon beside you so you can put the heat back in a teaspoon at a time.

Rice, iced tea and a plate of herbs all blunt it. A glass of tra da costs almost nothing and is on every table anyway. The language guide has the short list of phrases that make this kind of request easy.

When does Hue eat this soup?

In the morning, and the stalls that matter are done by mid-morning. The pot goes on before six, the queue arrives on the way to work, and a place that sells three hundred bowls has nothing left to sell at eleven.

The morning timetable is the single most useful thing to know before hunting for the dish. An evening search in Hue finds restaurants rather than kitchens, and a restaurant bowl is a reasonable dinner and not the same experience.

Hue itself is small enough to walk this. The things to do in Hue page sets out the mornings the city is worth getting up for, and this is the cheapest of them.

What does a bowl of bun bo Hue cost?

About 35,000 to 50,000 dong at a pavement stall in Hue, which at roughly 26,000 dong to the US dollar is under two dollars. The same bowl runs 50,000 to 70,000 dong in Hanoi, 688 km up the railway, and about the same in Ho Chi Minh City. Beef costs more in both, and rent costs more still.

Tourist-facing restaurants in all three cities charge two to three times that, and the bowl is usually milder rather than better. The cheap eating guide explains how to read the gap between a stall price and a menu price.

Cash is the norm at the stall end of that range. Small notes, as always — the money guide covers which denominations a breakfast kitchen can actually change.

What is the plate beside the bowl for?

The side plate carries shredded banana blossom, bean sprouts, herbs and a wedge of lime. All of it goes into the soup rather than onto a fork. Banana blossom is the local marker: pale, crunchy, slightly bitter, and almost never served with pho.

Add it in handfuls as you eat rather than all at once, or the broth cools and the last third of the bowl turns into a salad. The herb guide names what else tends to appear on that plate, which varies by stall and by season.

Lime goes in at the end, when the broth has already told you how salty it is. Squeeze it over the noodles, not into the chilli jar.

How does the soup change outside Hue?

Southern versions sweeten the broth and pile the bowl higher. Ho Chi Minh City cooks add sugar to the stock and hand over a larger plate of herbs. They also pile in cuts a Hue kitchen would not recognise as part of the dish: trotter, rolled pork, sometimes a crab cake.

Hanoi versions sit somewhere in between and dial the chilli down. Nowhere north of the central coast uses as much mam ruoc as Hue does. The further you travel from the city, the more the soup tastes of beef and lemongrass and the less it tastes of the sea.

Among the country's other regional soups, this one travels the best. The noodle soup guide covers the ones that do not, including cao lau, which is worth eating only in Hoi An.

Is there a bun bo Hue without the beef?

Hue cooks a vegetarian version, bun bo chay, and cooks it more seriously than most cities would. The old capital has a deep Buddhist kitchen tradition. On the first and fifteenth days of the lunar month, whole streets of vegetarian stalls open for people observing chay days.

The chay bowl replaces the stock with mushroom and pineapple, the beef with tofu and seitan, and the shrimp paste with fermented soy. It is a different dish honestly presented rather than an apology for one, which is not something every vegetarian option in the country manages.

The vegetarian guide explains how to find those stalls. The word chay on a sign is the only one you need.