It would seem that London's best burger is being served from a food truck. How do we know? It's not just the long wait—an hour and a half on a recent Sunday. It wasn't just the ringing endorsement by renowned food columnist Richard Johnson, who said of this particular burger that he'd “found something that bettered perfection.” The Meat Wagon's offering was just named “Best Sandwich” in a cook-off judgedby restaurant big-wigs Marco Pierre White and Mark Hix, among others.

Yianni Papoutsis, a former technician for the English National Ballet, first started offering his now-famous burgers in the summer of 2009 out of a tiny fiberglass van. After a few setbacks—some expected (ghastly London winter), some unexpected (a robbery)—Papoutsis finally drove onto the scene in April in a larger steel truck. The new Meat Wagon is outfitted with a cast-iron griddle that Papoutsis reckons is at least 30-years-old.

“I like to cook on blistering heat," he says, "and this gets really hot and stays really hot."

Papoutsis is just as obsessive over what goes into his burgers. His fresh-formed patties are minced earlier in the day from 28-day dry-aged chuck, trimmed to an 85/15 lean-fat ratio. He eschews the classic 80/20 rule. “It’s the holy grail for most people. But if you have big lumps of fat, you’ll just have big lumps of raw fat if you cook your burgers as rare as I like to cook mine,” he explains.

He may be open about his beef, but Papoutsis is tight-lipped when it comes to the cheese. He claims to have taste-tested hundreds of slices over a three-year period. One thing is certain—it’s definitely not mature cheddar. “That’s for show-offs," he says. "You end up just tasting the cheddar.” The ideal cheese complements the beef without being overwhelming.

Seasoned liberally with coarse-ground black pepper and salt, the patties and cheese are topped with bread and butter pickles and slices of red onion, and served on a lightly grilled bun. The final burger is juicy, flavorful, and messy—and heftier than it needs to be.

The Meat Wagon can often be found in Peckham in front of the Victoria Inn. This Friday through Monday, Papoutsis will be cooking at the British Street Food Awards market at Brent Cross. Get there early; at a food festival earlier this month, the burgers were sold out in just under three hours.

Ratha Tep is a guest blogger and frequent Food + Wine contributor based in Zurich.