London is the beneficiary of O’Sullivan’s good timing. After living and eating in Korea for two years he and partner Sarah Hogg, who’s from Scottish Borders, have rolled out their street cart just as sliders and Korean everything have emerged as want foods. They’re serving two different Korean-styled sliders (£2.50 each, £4 for a pair): lively but non-lethal kimchi with bacon and Korean-marinated pulled pork with crunchy sesame ginger cole slaw. O’Sullivan heats the prepared kimchi and pulled pork on his flat-top griddle. I tried both sliders and liked them enough to request a third – a mix-and-match pairing of kimchi and pulled pork.

Would I sulk if O’Sullivan eventually replaced the bacon with tender pork belly and crisp crackling? Think not. Would I lose sleep if his pulled pork got drippier over time? Duh! But for now, the only component that demands an emergency rethink is the drab, bready Sainsbury’s bun. It would help to toast or steam it on the griddle so that the slider felt warm and inviting as you transferred it from hand to mouth. But what’s really required is improved packaging with the character to complement the kimchi and coleslaw.

O’Sullivan and Hogg may be seeking a new home for their street cart following their royal wedding weekend gig at the Real Street Food Festival (Southbank Centre). You can track their movements on the Street Foodie website. But first their Korean sliders merit a new housing.

Oh God no! Have you tried it. It’s gross!

I sort of understood what he meant. Burgers, like cupcakes and meatballs, two other madly popular retro comfort foods of the moment, put you in touch with your inner 12-year old. That’s a big part of their appeal. And any non-Korean 12-year-old will tell you the very idea of fermented, pungent, spicy cabbage-on-a-cheeseburger is gross. Likewise a slippery, grease-leaking two-fisted burger that’s as hard to hold securely with its bun as is creme caramel with chopsticks.

What did you find gross, I asked Charlie, the kimchi topping or the makeup of the burger itself?

Kimchi. I thought it tasted of terribly strong blue cheese. SO disappointing.

If anyone in the Almond family, inspired by a Momofuko Milk Bar creation, had plans to bake kimchi blue cheese croissants for Charlie this Christmas I would suggest they reconsider.

The Hawksmoor kimchi burger is hardly faultless. The safely spicy kimchi is not crunchy, as any sort of burger pickle ought to be.  It’s runny rather than chunky – no help to an unctious steakhouse patty which, augmented with beef marrow on the inside and braised beef on the outside, is engineered to be sloppy.

Even so, a great burger is an untidy, implosive handful you shouldn’t eat without a stack of napkins at the ready. And by that standard, the thrill-a-second Hawksmoor kimchi burger is as great as it is gross.

The kimchi burger (£15 with triple-fries chips) is only available at Hawksmoor’s new Seven Dials location (near Covent Garden), for lunch in the dining room,  all day and night at the bar.