It’s the kind of serendipitous story that seems to epitomize successful relationships and businesses.

The way Taqueria del Sol owners Mike Klank and Eddie Hernandez tell it, Hernandez was working as a server on the day Klank opened his first restaurant, Azteca Grill, in Clayton County in 1987.

“I needed someone in the kitchen and Eddie told me he had experience cooking,” Klank says. “I said, ‘Go work in the kitchen.’ Then he became a partner, and the rest is history — which has been pretty good, because the partnership has lasted since ’87, which is over 25 years, now.”

That’s how Klank and Hernandez began the exploration of the nexus of Mexican, Southwestern and Southern cooking that would go on to become famous in the food world and lead to a string of local restaurants and franchise locations in several states.

“It was the right place and the right time,” Hernandez says.

Sitting at an umbrella table on the front deck at the Taqueria del Sol on Cheshire Bridge Road one afternoon, Klank, 63, and Hernandez, 58, regard the long line that snakes through the front door of the restaurant. From time to time, customers, many of whom have become friends, stop by to offer a greeting or shake a hand before joining the queue and the crowd inside.

The restaurant’s “system,” as the partners like to call it, was one of the earliest and best iterations of the “fast-casual” style that’s become common in the restaurant business. It requires customers to stand in a single line to order and pay for food and drink, before finding a table where runners deliver the goods.

Athens-based chef, restaurateur and “Top Chef” judge Hugh Acheson is an admirer.

“In fine dining, we look upon good versions of fast-casual with envy, sometimes,” Acheson says. “We have our systems down, but what they can do so quickly and effectively is pretty amazing to watch.

“You can barely get your ice tea before your food arrives. You wait longer at McDonald’s. And they’re not cooking from scratch, like Taqueria del Sol.”

Along with the signature dishes Klank and Hernandez have created over the years — Memphis-style pork barbecue tacos, spicy turnip greens, shrimp corn chowder — they are proud of the way the system works with their simple but imaginative, budget-priced food.

It’s a combination that’s garnered the partners three consecutive nominations for the James Beard Foundation Award for Outstanding Restaurateur, beginning in 2011.

Awarded to “a working restaurateur who sets high national standards in restaurant operations and entrepreneurship,” this year’s winner was New York City’s Maguy Le Coze of the iconic French seafood restaurant, Le Bernardin.

“Not bad for an old hippie,” is how Klank describes his climb to the company of the culinary elite that the Beard Award represents.

Klank, who favors a full beard and a uniform of jeans and button-down shirts, was born and raised in Memphis where he developed a love of barbecue and Southern food.

He came to Atlanta to attend Georgia State University and graduated with a degree in civil engineering. While he was in school, Klank started working in bars and restaurants, most notably Manuel’s Tavern, an Atlanta institution and neighborhood watering hole run by Manuel Maloof, who would go on to become a very colorful and prominent politician.

“When I first started working at Manuel’s, around ’69 or ’70, they just served hot dogs and sandwiches,” Klank remembers. “When you took an order, you went back, got the meat out of the cooler and sliced it, dressed the sandwich and served it.”

Klank went on to get an MBA from Georgia Tech. But the restaurant life had taken hold of his heart and mind.

“I liked working in restaurants, as it turned out,” he says. “It’s much better to work at a job you like because then it’s not work. You’re coming to work and having a good time.”

After spending time in Colorado and New Mexico, Klank became interested in Mexican and Southwestern cuisines, developed a taste for Hatch chiles and got a notion of the kind of restaurant he wanted to own and operate. After meeting Hernandez, that became more clear.

Hernandez, whose close-cropped hair, beard and black chef garb give him the look of a beatnik monk, was born in Monterey, Mexico, where he spent time in the kitchen with his mother and grandmother.

“My grandma owned a couple of restaurants and bars,” he says. “There was no set menu. It was whatever she wanted to do for the day. I loved watching her be creative with whatever she had, whether it was cauliflower or corn or goat.”

Nevertheless, “being a chef was never in my plans,” he says. Instead he hoped to become a professional musician. For a time, he hit the road as a drummer in a popular Latin band, but ultimately decided it wasn’t the life for him.

“It’s just not a good life,” Hernandez says. “I drank too much. I didn’t sleep at all. I figured if I stayed in the band I would die in five years.”

He started working as a chef in Texas, before moving to Georgia. Together, he and Klank made Azteca Grill into a place that proved popular with both critics and customers.

“It was a type of Southwestern food that wasn’t being served anywhere in Atlanta,” Klank says. “I was introducing New Mexico chiles and other products. Even Mexican restaurants were mostly Tex-Mex in those days. They didn’t use tomatillos. They didn’t make chile rellenos out of poblanos; they used bell peppers.”

In 1991, Klank and Hernandez moved into Atlanta and opened Sundown Cafe on Cheshire Bridge. That’s when and where their style — often dubbed “South-by-Southwest” or “Memphis-meets-Monterey” — began to take on a more experimental dimension.

The full-service restaurant served dinner only, had a full bar and featured jambalaya, shrimp and grits and ahi tuna, prepared with a potent array of peppers and spices.

Like Azteca Grill, Sundown was something of a critic’s darling, often appearing on best-of lists and gaining the favor of former AJC restaurant reviewer Elliott Mackle, who named the kitchen’s skate wing with brown butter sauce his favorite dish one year.

The roots of Taqueria del Sol and the seeds of the “system” can be traced back to the time when Sundown regulars began to clamor for lunch.

Sundown started doing fast casual lunch service in 1993, serving tacos three days a week. But by popular demand, that soon stretched to five.

In 1995, they tried another intown restaurant, Azuni Grill on Roswell Road, that didn’t work. In 2000, they created the Taqueria del Sol concept, fine-tuning the Sundown lunch system for dinner, keeping the full bar, and opening the first location on Atlanta’s Westside in 2001. A second one followed in Decatur in 2002. In 2005, they decided to close Sundown and reopen it as their third Taqueria.

Predictably, some of the Sundown regulars who loved stopping in for a quick lunch hated the system at dinner. “When we closed Sundown, people said, ‘You can’t do that.’ They were mad and some are still mad,” Klank says.

There’s a fourth Taqueria location in Athens and now Sol Catering, an event facility with a commercial kitchen.

Last year Klank and Hernandez took a big leap into franchising. It’s something they had previously considered a no-go, and they may still be wondering about the decision, even if they won’t say it out loud.

Currently, there are franchise locations up and running or coming soon in Florida, North Carolina, Pennsylvania and Tennessee. There’s even talk of opening in New York City’s Chelsea Market.

But an Orlando franchise that opened in October 2012 closed after just four months. And that begs the question: Is Taqueria del Sol the system and the recipes, or is it Mike and Eddie?

The partners work 50-60 hours a week, keeping their hands in at all the Georgia locations, testing and tasting recipes for new dishes almost every week.

“You have to want to replicate the system and the recipes,” Klank says. “It’s a synergy of everything that makes the whole thing successful. But how much of the restaurant is hinged around our personalities? That’s the question, I guess.”

Asked if they ever fight, Klank answers without hesitation: “Yeah. We yell and scream at each other. But we’ve never gotten into a fist fight or anything like that.”

“We fight about systems and people,” Hernandez says. “But if he doesn’t like who I have working in the kitchen, he’s going to have to live with that. And I may not like who he has working out front, but that’s his decision.”

From day one, Klank and Hernandez have done things their own way, despite resistance at times. There have always been those customers who try to buck the system by bolting the line and grabbing a table before their turn. And a common complaint is that Taqueria del Sol isn’t family friendly because there are no highchairs.”

None of that fazes Klank or Hernandez, who seem to enjoy being irascible as much as being creative.

“If you understand the system, you will always have a good time,” Hernandez says with a resolute grin.

“I’m a firm believer that you can’t be everything to everybody,” Klank says, sharing the grin. “And you shouldn’t try to be. I may have gotten some of that from Manuel. The first day I ever worked there, he said, ‘Boy, one thing you gotta remember, the customer is always wrong; we’re always right.’ I said, ‘No problem.’ ”