Julie Vu knew where to find the right baguette for her bánh mì sandwiches at City Càphê, the Vietnamese luncheonette she was planning to open this month in the City of London. She just wasn’t sure she could find it within 100 miles of Ironmonger Lane.

The baguette she grew acquainted with on forays to Vietnamese sandwich shops in the Belleville quarter of Paris was crackly but not hard, its thin crust giving way to an airy interior without the chewy, open-holed texture of traditional French baguettes. In London she sampled dozens of prototypes but despaired of sourcing anything exceptable until, voilà, she thinks she has it.

I wasn’t invited to Friday’s staff sampling of bánh mì in advance of the official opening on Monday (7 September). I just happened to be in the area, scouting burgermonday possibilities, when I spotted the City Càphê from afar. It’s amazing how my distance vision improves when the subject relates to coffee (càphê is Vietnamese for café). I entered the adorable shop and saw that Julie (left) and her friend Fiona Duong (center) were offering samples of their bánh mì.

The baguette, as viewed in this detail of sandwich cross section, looked bready to the eye, but its airiness was immediately apparent upon first bite. The interior compressed into the fillings as the flaky shell shattered in my mouth. Yum. Yes, I think she has it.

The three varieties of bánh mì I tried were already good, if a little plain, and likely to get better. With her baguette in hand Julie is prepared to think more about fillings and proportions as well as delve into more philosophical questions, such as: to mayo or not to mayo? Authentic or not I am fine with a brush of mayo on the baguette, as long as it’s just a coating and not the main ingredient. For mayonnaise sandwiches in which chicken, beef, pork, tuna and egg are merely accessories we already have Pret.

City Càphê – 17 Ironmonger Lane, London EC2V 8EY

How, I wondered later, could anyone get so close to that sandwich yet walk away empty-handed? I tried my very best to put myself in her Christian Louboutins and soon felt a great wave of sympathy wash over me: It wasn’t the 3-minute queue that put her off. Rather, it was the startling indifference of the scoundrels on it. My own misbehaviour, I would, if given the chance, assure her, was due less to bad manners than cowardice: I hated to think of the mean looks the other blokes would have shot at me had I, the first on the queue, let her pass ahead of me and therefore them, too. This was not the moment to make nice like Mr Knightley.

When the uninitiated are asked by an O’Shea’s butcher which of the available sandwich accessories – English mustard, apple sauce, rocket (arugula), etc.– they want on their pork belly they can be forgiven for thinking they actually have a choice. In reality the only thinkable option is, (D), all of the above. You would have to bring together David Crosby, Steven Stills, Art Garfunkel, Brian Wilson and Stuart Murdoch in their respective primes to record lush harmonies comparable to those in that sandwich. The condiments melt into the succulent pieces of meat within the warm embrace of the oven-fresh baguette.  And lest the ensemble become too mushy there is the percussive crunch of delectable cracking cut into long, thick strips to break the melody line –  and possibly your incisors, too.

Sadly the pork belly baguette is not a daily lunch affair. Darragh O’Shea rotates his sandwich meats to, as the sign says, “mix things up and keep you happy :)” Keep you happy? Apparently they are unaware of the anguish felt when walking out of this Knightsbridge butcher shop without a pork belly baguette.

11 Monpelier Street, Knightsbridge, SW7  (see map) – 020 75817771