A single dish had filled the entire restaurant. The Tongluowan branch of the century-old Qingheyuan chain in Shanxi province's capital, Taiyuan, was packed on Jan 4.

People were lining up to savor a traditional soup with more than three centuries of history－tounao.

Wang Sanfu took his granddaughter to Qingheyuan on the first weekend of 2020 to enjoy the soup that the 83-year-old recalls first trying in 1959.

Tounao is a creamy broth made with lamb, yellow liquor, lotus roots, Chinese yams and other ingredients like Chinese herbs. It's consumed alongside marinated Chinese chives.

It's Taiyuan's most-celebrated breakfast, not only at traditional local eateries like Qingheyuan but even at the city's Sichuan and Hunan restaurants.

It's said that physician Fu Shan created the dish over 300 years ago, explains Wang Hemao. Wang is a fifth-generation inheritor of tounao-making skills, which are recognized as a provincial-level intangible cultural heritage form.

"Tounao was originally called eight-ingredient, or yimu, soup. The second name means 'to help mothers' health' because Fu created it using eight ingredients to improve his mom's health," Wang says.

It's said that Fu's mother became healthier after she started eating the soup regularly. Neighbors asked for the recipe, which Fu also shared with the owner of a lamb-soup restaurant that was later named Qingheyuan.

"It looks like a simple bowl of soup. But the process is actually quite complicated. There are at least a dozen steps."

The yellow liquor is made using millet, which takes 20 days to ferment. And the flour must be steamed for at least three hours.

Qingheyuan's tounao cooks clock in at 5 am. They first make the base with steamed flour, mutton and yellow liquor. The meat, Chinese yams and lotus roots must be pre-cooked the day before.

"It's quite nutritious and good for your stomach. And it's best if you drink it in the morning," Wang says.

Taiyuan residents used to go for tounao before sunrise. So, restaurants that served the dish would hang red lanterns－a tradition that remains.

Elderly people in particular appreciate the liquor fragrance. Some drink bowls of yellow liquor along with the soup.

Wu Fusheng has joined friends to eat tounao for breakfast at Renyili restaurant three to four times a week from October until March for nearly 20 years.

"With a bowl of tounao and yellow liquor in your stomach, you feel warm and comfortable in the wintertime," Wu says.

"If you eat a bowl of tounao in the morning, you don't feel hungry at noon."

He considers enjoying tounao in winter to be a healthy form of leisure.

Zhai Dehou says he has heard about the dish frequently since he moved to Taiyuan at age 14. The 87-year-old can recall the three locations Qingheyuan has moved among over the years.

The restaurant offers a deal for diners over age 70, Tongluowan branch manager Gao Zhenrong says. They can get a bowl of tounao and a basket of shaomai (flower-shaped steamed dumplings) for 25 yuan ($3.6).

The dumplings have thin wrappers, and are filled with lamb, beef and leeks."They pair well with the soup," Gao says.

He says plunging temperatures lead to surging orders. His restaurant can sell between 800 and 1,000 bowls a day, and up to 1,500 at most.

"The price of lamb has increased from 44 yuan to 70 yuan per kilogram. But we haven't raised the price of our tounao or skimped on the ingredients," he says.

Gao believes the fact that many customers travel dozens of kilometers for a bowl shows how important tounao is to Taiyuan's residents.

"It has a special flavor that lingers in the memories of Shanxi's people," he says.