Rome wasn’t built in a day. And neither, it seems, was the Tommy’s Margarita.

Perhaps more than any other modern classic, this simple twist on the original—which showcases the tequila by eliminating the Curaçao and adding agave syrup as the sweetener—came together by accident, over the course of a decade.

The seeds of the drink were planted when Bermejo was not yet of drinking age. His early experiences with beer, rum and brandy left him with bad hangovers, but he found that tequila—filched from Tommy’s Mexican Restaurant, his family’s establishment in San Francisco’s Richmond neighborhood—didn’t do quite as much damage. And, he recalls, Herradura tequila in particular, made of 100 percent agave, left his brain largely unscathed.

Years later, in the late 1980s, Bermejo took up his post behind the bar at Tommy’s. He drew on that misspent youth, and switched out the mixto tequila being used in the house Margarita and replaced it with Herradura.

“Our pour cost went up dramatically,” he recalls, “and the drink’s cost only went up, like, 50 cents.” But for Bermejo, the change in flavor made it worthwhile.

Though the Tommy’s Margarita is famous for containing no Curaçao—a defining ingredient in a Margarita—eighty-sixing that liqueur was a decision Bermejo never had to make. By the time he arrived, it had vanished from the recipe used at Tommy’s, probably for economic reasons, and had been supplanted by simple syrup.

Replacing the simple syrup with agave syrup, however, was Bermejo’s idea. “Agave syrup was a product used mainly by California health food producers,” he said. “But even though it was expensive, it was a no-brainer for me. It was a product from a similar plant as tequila.” He also traded up from sour mix to freshly squeezed lime juice, and discouraged the use of a salted rim, thinking the flourish unnecessary.

Over the years, Bermejo began favoring Margaritas served on the rocks over the blended variety, the better to taste the tequila. By the end of the 1990s, Tommy’s Margarita orders had gone from being 95 percent blended to 95 percent on the rocks.

At the same time, he was beefing up the selection of 100 percent agave tequilas at the bar; by 1999, he carried 110, an astounding number even today. And he was slowly educating his clientele on the virtues of the spirit through “tequila clubs,” in which participants earned credits toward membership with each new bottle they tried.

For all his innovations, however, Tommy’s Mexican Restaurant was still in Richmond; Bermejo was working in a vacuum. “We were kind of in our own little world,” he says. “At that time, in my 20s, I was not out and about. We had no publicity. Our focus was just to make our guests happy, so they would come back.”

Then one day in 1995 or 1996, the greater bartending world—in the form of Tony Abou-Ganim, one of the most celebrated bartenders in San Francisco—found Bermejo. “I had never heard of him,” remembers Bermejo. “He came because his girlfriend was a customer and brought him.” When Abou-Ganim became the beverage director at the Bellagio Hotel & Casino in Las Vegas in 1997, Bermejo was brought on as a consultant. By then, Tommy’s was beginning to get regular local press about their tequila selection. In 1999, The Wall Street Journal published a large article on tequila, naming Tommy’s “the epicenter” of the spirit’s revival. The piece cemented Bermejo’s reputation as a tequila expert.

Around the same time, up-and-coming San Francisco bartender Dominic Venegas dropped by Tommy’s with his family and was bowled over by the house drink.

“I remember thinking how delicious the Margarita was,” he said. “There wasn’t any cloying citric flavor or salt hiding bad tequila. It was a revelation. Fresh lime juice!” When Venegas worked at Bacar in 2001, the Tommy’s recipe became the house Margarita. When Bermejo and bartender Jacques Bezuidenhout opened the bar Tres Agaves in San Francisco in 2005, the Tommy’s recipe was likewise employed as the house Margarita.

The very last thing to come into focus was the drink’s unassuming name—and Bermejo had nothing to do with that. Instead, he credits two British bartenders, Henry Besant and Dre Masso, with its inadvertent christening.

In 2001, Bermejo traveled to the United Kingdom and continental Europe with the Tequila Regulatory Council to aid the organization in getting the spirit officially recognized by the European Union. Bermejo’s job was to connect to the English-speaking bartenders. In London, he met Dre Masso, who was working at the white-hot LAB bar.

The next year, Masso asked permission to visit Bermejo; he spent six months in 2003 working at Tommy’s, listening to Bermejo’s stories and tasting tequila. “It was at this point that I discovered his house cocktail,” said Masso. “When you looked around the bar and restaurant it seemed every guest had a sipping tequila in one hand and a Tommy’s in the other.”

Masso introduced the drink to Besant (who died in 2013) and the two became the drink’s Johnny Appleseeds, telling colleagues about it and talking it up at cocktail demonstrations as founders of the Worldwide Cocktail Club, a consultancy they formed in the mid-aughts. Noted London mixologist Wayne Collins learned of the drink from Besant and soon began using the recipe in his own training sessions. Masso and Besant even opened a Mexican restaurant and tequila bar in London called Green & Red and co-authored Margarita Rocks in 2005, which includes what may by the first published mention of the Tommy’s Margarita. The name came easy.

“The Tommy’s [Margarita] was so synonymous with Tommy’s Restaurant that in my mind it has always been called that,” explains Masso.

Masso, Besant, Collins, Abou-Ganim and other Tommy’s evangelists did their job well. Since the mid-aughts, the Tommy’s Margarita has appeared on menus in Turkey, India, Indonesia, Colombia, Scotland, Germany, Ireland and many other countries. In 2014, a Denny’s in Manhattan was serving it.

Bezuidenhout vividly recalls encountering the drink in a small hotel in New Zealand. “That’s the day it hit me,” he says. “I had been seeing it in London and around the U.S. a lot. But this was this tiny bar in remote New Zealand, and they had it on the list.”

Bermejo himself has encountered the drink firsthand in Moscow and Cape Town, South Africa, where he was traveling with his mother in 2015.

“I walked up to the bar,” recalls Bermejo. “I said, ‘Can I get a Tommy’s Margarita?’ The bartender goes, ‘Sure!,’ in a South African accent. My mother looks at me and almost goes white.”