The wine spigots at the cash register—men popping in to fill tiny glasses with the house white, then ambling on. The construction workers making a quick lunch of exquisitely crisp-plump fried anchovies. The bickering between mother (in the kitchen) and son (at the counter) during the rush to serve regulars beneath the white stone arches of their 10-table trattoria.

It’s all a bit much, really. A Central Casting cliché that feels unreal. But La Cantinetta is real. And lunch at this convenience store/enoteca/eatery—where cat food and wine and potato chips and packages of pasta share shelves—has changed little since Maria Notaro took over more than 60 years ago. It also happens to serve some of the best food in Naples.

Not that you’d easily find it. It’s hidden on the city outskirts on a residential street on the wrong side of the tracks of Stazione di Napoli Centrale. I almost walked past, wine and water bottles stacked unceremoniously outside its double-wide doors, but for its sidewalk menu board. Scrawled casually were the words I’d spent more than a week searching for.

Zuppa forte.

I’ve eaten my way across Italy dozens of times, but never had I encountered a mystery the likes of zuppa forte, literally “strong soup.” A translation that does poor justice to this pasta sauce-bruschetta topping-­soup mashup that everyone I encountered knew and loved yet somehow knew little about. How is it made? What are the ingredients?

Non lo so. Chiedi al macellaio. I don’t know. Ask the butcher. I heard it over and again.

I learned bits and pieces along the way, but nothing that completed the picture about a dish best described as a highly concentrated, intensely flavored, gently spicy and deeply meaty tomato “sauce” that verges on paste and also happened to be one of the best things I’d ever tasted in Italy. Hence, my mission to master its secrets.

Stumbling upon Notaro’s shop, which she runs with her son, Luigi Tufano, on my last day in the city felt like a windfall. The octogenarian welcomed me into her kitchen, where she quickly boiled spaghetti until al dente, then combined it, some of its cooking water and a generous scoop of zuppa forte, tossing it all until perfectly wedded.

It was a rich, savory, spicy, meaty, sweet explosion. This is the Italian meat sauce we never knew how deeply we longed for. Importantly, there was a depth to it, something I couldn’t quite identify and had never tasted in an Italian pasta sauce. There was a complexity that I didn’t quite understand but that insisted I keep eating.

Before meeting Notaro, I’d spent a week scouring the city to learn the origins of zuppa forte, also called zuppa di soffritto. The tomato paste-looking sauce—deeply red, incredibly thick and speckled with small hunks of meat—shows up on almost no menus, but can be found at nearly every butcher, big bowlfuls sitting in the deli case, sold by the scoop.

But have you tried interviewing Neapolitan butchers? From most, I got at best partial answers about offal, tomato paste and lung meat. Some said it was cooked for days, some for hours. Ingredient list? What’s that? Most directed me to random restaurants “down the street,” where I reliably encountered answers equally sparing.

My first real clues came from Salvatore Giugliano, chef at Mimì alla Ferrovia, a restaurant his grandfather opened in 1944. He modernizes classic Neapolitan recipes, and though zuppa forte isn’t on his menu, he’ll make it for diners who ask for it. “The generation of my father, it’s part of their story,” he says. And he enjoys reviving it. “There’s a new generation of guys who are interested in recipes like this, the return to the old dishes.”

Zuppa forte, it turns out, dates to at least the 1800s and is a classic example of the region’s cucina povera, cooking that made the best of whatever was on hand. The poor worked the land, and the noblemen fed them the offal and other less desirable bits of meat. Combine that with tomatoes—this is pomodoro country, after all—and you’ve got a great sauce.

Though times have changed, traditionally it was a long-simmered sauce, as most in this region were. Men and women worked the fields, meaning dinner needed to simmer low and slow all day. Thus “strong soup,” a highly concentrated sauce that packs intense flavor and is just as good spread on toasted bread as diluted and tossed with pasta.

All of which made sense. But it didn’t explain that ... something deeper I could taste but not identify. Giugliano shrugged. Salsa pepperoni, of course. Of course...

Fermented chili paste, the key ingredient in zuppa forte. A thick paste of cooked and pureed sweet and hot chilies that is then fermented. A classic, if often overlooked, ingredient of the cooking of Campania. It adds that funky, savory-sweet depth to everything it touches. Much like Japanese miso and Korean gochujang.

In fact, when I finally tracked down salsa pepperoni—apparently it is sold, even to butchers, only from shops that sell cod ... because, why not?—and tasted it on its own, I found it almost identical (though a bit less complex) to gochujang.

Thankfully, Notaro was happy to walk me through her recipe. It indeed involved lots of tomato paste and salsa pepperoni and offal. Also ample bay leaves, garlic and rosemary. And plenty of simmering. All fine, except the offal. Americans’ appetite for offal, and its accessibility, is limited.

When I voiced this concern to Tufano, he shook his head dismissively. He loves his mother’s zuppa forte, still made with all those spare bits and pieces. But times are changing. People today want different cuts. Do you think people in the 1800s would have used offal if they could have used pancetta or other premium cuts? No! The spirit of this recipe, he said, it to use what you have. So use what you have!

So Tufano set our course for recreating this Neapolitan classic. Offal was out, but deeply flavorful pancetta was in. Salsa pepperoni can be challenging to find in the U.S., but gochujang has become widely available. Now add tomato paste, bay leaves, rosemary and garlic, and you have a very respectable version of simply the best pasta sauce I’ve ever eaten.