According to Liu Zheng, who is in charge of the restaurant, his team has spent months developing the soup base and a homemade chili sauce, which can be added to the soup based on diners' tastes.

"We've tried different combinations of rice in the fermenting process to make sure the soup is not too sour and has a sweet aftertaste. In the end, we found our recipe, which uses seven or eight kinds of rice," Liu says, adding that the chili sauce has also been fermented to take the edge off the spiciness.

Liu says that, for the sake lees soup base, anglerfish, puffer and beef are a good match, and they burst with flavor after boiling in the soup. Regardless, the restaurant still offers a special dipping sauce.

As the hotpot is new to customers, Liu says the servers will usually teach diners the best order in which to boil the ingredients and explain the features of each fresh fish.

Besides restaurants, dregs vinegar hotpot has also caught the attention of retailers. At the end of September, Alibaba-owned grocery chain Freshippo launched a dregs vinegar hotpot in around 300 branches across China, including Beijing, Shanghai, Hangzhou of Zhejiang province, Xi'an of Shaanxi province, and Guangzhou and Shenzhen of Guangdong province.

It didn't take long to get popular, and sales in the second week increased 305 percent over the first, ranking second among all flavors of hotpot base soup on the platform.

According to Zhao Jianjian, who is in charge of Freshippo's hotpot business, the reason that the dregs vinegar hotpot was launched was to save customers the cost of flying to Hainan, enabling them to enjoy the dish at home.

"Compared to Tom Yum Kung and other spicy and flavorful soup bases, the taste of dregs vinegar is simple. It does not take away from the taste of the ingredients boiled in the soup and at the same time, it absorbs some of their freshness and sweetness, turning into a rich soup with layers of flavor," Zhao says.

Zhao says that their team took two months to research and develop the soup base.