"Get fat." It's the Mary's Burgers motto. In an era of #wellness, it's refreshing to see something so unashamedly, well, body-positive.

The Newtown bar-restaurant serving arguably Sydney's best burger is where it all started for Jake Smyth and his business partner, Kenny Graham.

Since opening their first bar-restaurant in a former Greek Orthodox church-turned-sexual health clinic in Mary Street, the pair has opened a city Mary's, two pubs – the Lansdowne and the Unicorn – and Enmore bottle shop P&V.

But Smyth wasn't always the Grand Lord of Sydney's inner west. Once upon a time he was the teenage son of two Pentecostal ministers living in a small town in Hobart. He lived there until he was 14, when his parents moved the family to the industrial town of Singleton, in the Hunter Valley.

It was a hard four years for Smyth, and in year 9 he left to be home-schooled. But he didn't do a lot of conventional studying. Instead, he read, listened to a lot of music, and worked part-time at McDonald's.

When he was 18 he did a bridging course and spent a year at Newcastle University. But tertiary study didn't agree with him either.

More fat, more acid, more salt. That's it. If something's a bit flat, add these three things and you'll be fine.

He took off to Britain, where he met his now-wife, Ali, on a London bus, and also his business partner, Graham, while tending bar in Edinburgh. Smyth and Graham soon became firm drinking buddies. "We just bonded over our shared hatred of things in the hospitality community. At that point I'd never met somebody who was as equally passionate about hating things as I was. That's how we started. Just dreaming up ways in which we were going to mess with shit. How we could improve things and cut through the bulls---."

When Smyth moved back to Australia, he and Graham stayed in touch. Graham, who had an Australian girlfriend (now wife), moved over not long afterward and the pair started working for the Porteno Group. "Now, we're just two old fat f---s hanging out together."

Growing up Pentecostal was all about outreach and creating community. But it wasn't a community Smyth connected with. He eventually found his tribe in restaurants and hospitality. Now a staunch atheist, he chose to make Sydney his religion. "There's a line from one of my favourite bands, Gang of Youths. They say, 'Say yes to sticking with a city through a thousand days of rain.' And you know what, I get emotional because of that. This is my town, I love Sydney."

So back in 2014, Smyth made Mary's his church and the inner west his parish.

"We never wanted to be the busiest or best, or most successful. We just wanted to open a bar that served the food that we loved, and listen to the music that we loved, and serve the drinks that we loved. We wanted to put roots down and open up somewhere that our kids would inherit. Our interest was solely in being excellent at what we did every day. It just so happened that we sold burgers. And we got really lucky with that part."

It's a very different trajectory to the one he was on a decade ago. Back then, Smyth was a young waiter working at Glebe Point Diner under restaurant manager and service legend Andrew John (ex-Sean's Panaroma). It was here Smyth first became interested, not just in the art of great service, but also in food and cooking. "I was the world's best restaurant manager for any chef," he says, "because I cared about the kitchen. I'd pull [the dish] apart. [I'd ask] 'How are you doing this? Why do you do this? What happens here?' I loved cooking and the mystery of how to achieve it."

Smyth also spent time at Glebe Point Diner's short-lived offshoot, Neutral Bay Diner. He's the first one to say the diner wasn't well suited to the neighbourhood.

"I did my best, and I tried really hard. Honestly, they wanted me over there because I was very representative of Glebe and they wanted to capture that. But the venue didn't really translate. Even as we look to expand Mary's over the next 12 to 18 months, one thing that will stick really clearly with me from my time at Neutral Bay is there's no such thing as picking up a venue and sticking it in another suburb."

Smyth ended up at Bodega, the hottest prospect in Surry Hills in 2013, where he was a natural fit. It was here, while learning about the inner workings of kitchens, that he and Graham fleshed out the concept for Mary's. "I was basically getting trained by these guys without ever actually working in a kitchen," says Smyth. "When it came to building flavour [chefs Ben Milgate and Elvis Abrahanowicz] are the ones who first broke it down [for me]. "More fat, more acid, more salt. That's it. If something's a bit flat, add these three things and you'll be fine."

These days, Smyth is reluctant to describe himself as a publican. He's not quite a bartender or a restaurateur. In fact, he's not quite sure who he is on paper. But he knows what he wants to do. And that's to create a legacy, especially when it comes to his pubs.

"To really create a pub worth its weight, it's all about time. I want to have a pub like the Shakespeare [a Surry Hills institution]. I want my kids to go there and pour pints behind the bar of a pub that was lived in, and that had its own proud history. I think a community springs up around a good pub."

Quickfire corner

After-midnight snack: Double cheeseburger on a steamed bun, and a Big Mac on a steamed bun with extra cheese and extra sauce on it.

Indispensable work tool: My singing. I sing at everyone, I sing abuse, I sing praise, and I sing frustrations.

Formative food moment: It was an amuse-bouche at a Michelin-starred restaurant in Edinburgh called Restaurant Martin Wishart. They sent out pumpkin soup with puffed rice. And it was the most incredible pumpkin soup I'd ever had. It was amazing, just the most incredibly complex, most pure pumpkin flavour I'd ever had.

Secret super power: Being able to wake up hungover, look after the kids and still turn up to work on time.

Music to cook to: I like LCD Soundsystem to cook to. If I'm cooking at home for Ali and it's a quiet night, Radiohead. If it's for a party, Ryan Adams. I like to listen to music that I can sing along to. That's what I do. I cook and I sing. And I just sing at the top of my lungs, you know, at my daughter or at my sous chefs. Whatever.