Yiguan Gu has a secret.

Every other day at 6:30 in the morning, when the halls of the Krog Street Market in Atlanta are desolate save for a few early shopkeepers prepping for the day to come, Gu unlocks the door to the kitchen of his family's counter restaurant, sets a big stainless-steel on the range, and starts up a batch of his secret Zhong's dumpling sauce going.

What, exactly, is in it is a mystery even to his daughter, Yvonne Khan, who he's training to take over the business one day. The salty-spicy-sweet liquid is the glossy black-brown of burnished mahogany, and has the consistency of an olive-oil dressing. But after he takes it off the flame, the sauce will have reduced so much it'll have be a full hand's breadth lower down in the pot.

There are, of course, clues to his secret. Gu admits that he adds homemade chili oil to it, and that there a total of ten ingredients—the earthy sweetness of Chinese plums pokes out its head, for one—but Gu's the only person in the world who knows precisely what goes in the dumpling sauce, in what order, or when.

But Gu will happily tell you what the single most important thing to go into the dumpling sauce is.

"Time," Khan says, translating from the family's native Szechuan dialect. "It takes a long, long time."

Gu was born in Chengdu, China, in the Szechuan province, famous around the world for its fiery cuisine. His mother was a cook, and when Gu was too young to walk, she strapped him to her back and worked in the kitchen of neighborhood restaurants with him still attached. He began cooking himself when he was about waist-high, and there never seemed to be any doubt that Gu was destined to send most of his life in a kitchen. He wrote about Szechuan cuisine in local publications, trained with the area's culinary institutions, and set out to master the province's history-laden menu of food.

But there was one dish Gu knew he had to perfect. Zhong's dumplings has been around for centuries, and is a deceptively simple dish—flour skins filled with a tablespoon of minced pork, and a handful of seasonings including ginger, salt, soy sauce, salt, eggs, and water. But in Chengdu, there was one restaurant that stood above all the rest—a tiny shop that made Zhong's dumplings and nothing else. Gu had eaten there as long as he could remember, and there was regularly a 30-minute wait there. The dumplings were delicious, but the owner's sauce was heavenly. Gu, then 22, needed to learn how to make that sauce.

And after months of pleading and, critically, the intervention of Gu's mother, the owner finally gave up.

"He said, 'Fine, I'll teach you, but only when the restaurant's closed. And you can't tell anyone,'" Gu says.

Decades later, after having moved to the U.S. and worked at Szechuan restaurants around the country, Gu finally opened his own restaurant, on Buford Highway in Atlanta . His plan was to open a place that catered to the area's Chinese population, but he was worried that Americans wouldn't like the spiciness of the Szechuan cooking, or the location away from the city center.

"The first day we had three people," Khan says. "My father's friend said, 'Don't worry. With your cooking, the location won't be a problem for long."

It wasn't. Within weeks, word of mouth about the new place, with its loyalty to authentic Szechuan cuisine and run mostly Szechuans, got out. The clientele, at first only Szechuan then other Chinese, then Korean, Japanese, and Indian, broke into the Atlanta mainstream, and Gu's Bistro became pilgrimage point for Szechuan lovers of all backgrounds. Every weekend filled up with 100 reservations and one- to two-hour lines.

"We kept having Americans coming in asking, 'More spicy! More spicy!'" Khan says. "My dad was like, 'Those Americans actually really like spicy food!"

And the star of the menu? Gu's Zhong's dumplings and his secret sauce. The restaurant sold 200,000 dumplings a year.

"Everything has to be measure exactly right, so that the filling is not too soft, not too hard," Khan says. "When he mixes it, it's like exercise—he sweats the faster he makes the food."

And the sauce?

"We see people drink the sauce from the bottle."

Gu's Bistro closed on March 2, 2015. Twenty-two days later, the family reopened in Krog Street Market as Gu's Dumplings, a food stall without seating that focused on those dumplings and was closer to the city center. It was an idea that Khan had pushed as the rent on the Buford Highway location kept rising.

"No one has an authentic Chinese-dumpling place in town," Khan says. "We loved the concept of the market—people can go to the bar and grab a drink, then meet with friends or go on a first date, and take advantage of the fact that every place in here is different but they're all one of the best—the best burgers, barbecue, Yalla's, Gu's Dumplings. I said, 'Dad, don't worry. As long as people like food, they'll love your food."

For now, it's still Gu's food, and his alone, as long as that sauce recipe remains locked in his skull. But he promises that the tradition will be passed on to the next generation—when his daughter is ready.

"Now he's teaching me how to make soup and the side dishes," Khan says. "Slowly, very slowly. He says, 'You have to follow my rules, and you can't change my recipe, but at the end of this journey, I'll pass my experiences on to you, and then you can do the same for your kids.'"

Khan grins widely.

"I'm a very good student."