Despite none of them having any experience of running restaurants, and yet to graduate, in October 2006 the then 22-year-olds started on their business plan in their college dormitory.

Their idea was to set up a fast-food restaurant that only sold bowls of salad, both cold and warm ones, such as oven roasted vegetables. And instead of buying their produce from supermarkets or wholesalers, they would source directly from local farmers to ensure everything was as fresh as possible.

By the summer of 2007, after they had graduated, the three friends had raised $300,000 (£233,000) from friends and family. In August of that year they opened the first Sweetgreen restaurant in an abandoned pub in the Georgetown area of the US capital.

“We just kind of opened the doors,” says Nicolas, whose job title is chief concept officer. “We had hired a few students, but we hadn’t trained them that well.

“The first day was pretty slow, but it felt very busy because we were testing our system, and were moving very slowly. We got busier every day after that.”

Jonathan, who is the chief executive, says that to begin with they had no plans to open more than the one restaurant. “We didn’t think it would turn into a career,” he says.

“We just saw it as a way to solve a problem, because there was such a need for healthy food. We thought we would open one restaurant and then do something else.”