The success of a Japanese man who took an "onigiri" rice ball sandwich known for years as the soul food of Okinawa and turned it into a booming franchise began when his wife cooked the pork and fried egg snack for him one morning at home.

Shortly after his culinary revelation about a decade ago, Katsuaki Kiyokawa, 53, started a restaurant specializing in the pork tamago onigiri sandwiches, and his chain Potama Co. has since grown rapidly beyond Okinawa to big cities like Tokyo and Fukuoka, as well as overseas to Hawaii.

Katsuaki Kiyokawa, president of Potama Co., poses for a photo holding a pork and fried egg "onigiri" rice ball, in Fukuoka on Dec. 11, 2023. (Kyodo)

Rival shops have also sprung up to take advantage of the craze for the hot and fluffy treats, inspiring the food industry in Okinawa to try expanding the appeal of other local cuisines and specialties.

Pork tamago onigiri are made of simple ingredients: grilled Spam, the canned meat which has become a common sight in countries around the world, and fried eggs wrapped in rice and dried seaweed.

Like fast food, they can be easily held in one hand and eaten on the fly, contributing to the food's allure for people inside and outside of Japan.

Spam, a processed can pork and ham product introduced in 1937 in the United States which became a central part of the U.S. military diet during World War II, was absorbed into the local Okinawan diet during the Allies' occupation following the war.

Long a staple food and home-style meal in Okinawa, major convenience store chain FamilyMart Co. began selling pork tamago onigiri in the prefecture in April 2000. Now, it is widely sold at "obento" take-out lunchbox shops and convenience stores.

In October 2022, a Tokyo-based information technology firm began a delivery service featuring the hearty snack. A recent onigiri boom has also helped boost its popularity.

Potama, based in Okinawa's prefectural capital of Naha, runs 11 outlets at home and abroad and is rapidly expanding its operations. Kiyokawa was born in the Osaka Prefecture city of Sakai, western Japan, but in 2000 he relocated to Okinawa where he began operating a cafe.

He married a local woman in 2013, and it did not take long for him to change his fortunes after being blown away when she showed him how good a freshly home-cooked pork tamago onigiri could be.

To put it mildly, it was the most delicious pork tamago onigiri he had ever tasted. "I told her, 'Wow, I never thought a freshly cooked one could be so tasty,'" he said.

Wishing to share his delight with as many people as possible, Kiyokawa began buying up all kinds of pork tamago onigiri at lunchbox counters and shops to research how to make the best version. Settling on Spam as the most recognizable of luncheon meats, he explored the best way to fry eggs and the optimal amount of rice to use, among other elements.

Kiyokawa, who calls his onigiri sandwiches "a feast in your hand," opened his first shop in Naha in 2014. Being a stickler for freshness, he only cooks enough pork and eggs to be used up in five to 10 minutes, meaning everything is prepared upon order.

The shop became so popular among tourists through word of mouth that lines began to form outside before it opened each day.

Potama set up its second shop at Naha Airport in May 2017 before launching operations in Tokyo Midtown Yaesu, a shopping mall complex connecting directly to Tokyo Station, as well as another outlet in a bustling shopping district in Fukuoka city in Kyushu, the southernmost of Japan's four main islands.

Potama opened a store in Hawaii in 2020 and has since seen a positive response from locals in the region who were already big Spam consumers, with another Japan-inspired dish -- Spam musubi featuring similar ingredients -- already popular.

The company's sales are estimated to have exceeded 900 million yen ($6 million) for 2023, a record high.

In mid-December, Yuri Ushijima, 35, a local who patronizes Potama's Akasaka shop in Fukuoka -- one of its three shops in the city -- once a week, said, "The onigiri is sized just right, and it's always warm and tasty."

Kiyokawa's policy is to offer a unique menu at each store. For example, the Akasaka shop and the one at Aso Kumamoto Airport in Kumamoto Prefecture use local specialties of "aburi mentaiko" (roasted cod roe) and "aka ushi" (red beef) steak, respectively.

His specials also include goya (bitter melon) tempura, shrimp mayonnaise, taco rice and garlic butter steak, among many varieties.

Upon entering the Tokyo market in 2022, Potama developed a pork luncheon meat made from 100 percent Okinawa-raised hogs.

By gradually expanding the use of the meat, Kiyokawa hopes to increase consumption of Okinawan products and help stimulate the local economy.

"I want to contribute to Okinawa, a place that helped me succeed though I was an outsider," Kiyokawa said. "Our hope is to spread Okinawan food culture around the world."

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