America's obsession with pork notwithstanding, the porchetta sandwich is lauded by many as the best thing to happen between sliced bread. The holy grail of the porchetta sandwich, most foodies say, is only available in one place: Roli Roti at the Ferry Building farmers market in San Francisco. It's the first food my pescatarian sister urged me to try when I visited the fair city, and suffice it to say she did not steer me wrong.

So what is a porchetta sandwich, and what makes the Italian sandwich so great?

Italian Pork Roast

The sandwich begins with no ordinary pork roast, but a porchetta, made according to a classic Italian recipe. The porchetta layers flavors and textures from the inside out, starting with the white-wine paste containing garlic, lemon zest, and other seasonings that's spread on the inside and outside of a butterflied pork roast. The roast is then rolled tightly—which keeps the meat juices inside—and wrapped in a layer of pork belly. Once it's tied up, it's roasted until that belly is a crisp counterpoint to the fatty, luscious meat.

Each slice boasts a swirl, then, of tender pork, bright seasonings, and the crunchy umami of the pork skin. And while it's good enough alone, it's even better in a sandwich.

Sweet and Spicy Toppings

What goes on a porchetta sandwich typically marries the flavors of the pork roast with something sweet and something spicy. At Roli Roti, they spread a sweet onion marmalade on it, house-cooked slow and low, over 12 to 15 hours. That musky sweetness contrasts the bite of curly cress or arugula, atop a roll soft enough to cushion the concoction, but crusty enough to withstand the juices that flow from it.

Where to Try One

If you can't make it to San Francisco's Roli Roti, look for a porchetta sandwich at an Italian sandwich shop near you:

• At The Publican in Chicago on Saturdays only, they serve a porchetta sandwich with bread and butter pickles and whole grain mustard on a pillowy ciabatta roll

• The Foodery at Rittenhouse in Philadelphia crafts a classic East Coast iteration, topped with broccoli rabe, "italian long hots" (hot peppers), and sharp provolone

• At the aptly named Porchetta in New York City, you can also get the sought-after roast with beans and greens or as part of cubano

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