When it comes to seafood, we all have our favorites. It might be maguro (tuna) sushi, charcoal-grilled unagi (eel), a salmon steak or a nice fillet of batter-fried plaice. For Mai Nagamatsu, though, there’s no question: Her fish of choice is katsuo (skipjack tuna).

That’s obvious from the name she’s given her breakfast counter, Katsuo Shokudo (“Skipjack Diner”). But don’t roll up expecting platters of gleaming sashimi, tataki (seared skipjack) or even side dishes of the cooked fish. Nagamatsu’s focus is on katsuobushi, the cured blocks of skipjack that have long been the prime source of umami seasoning in Japan.

The traditional process by which the blocks are made is long and labor-intensive, involving heat, smoke, mold and considerable expertise in fermentation to turn raw fish into concentrated umami goodness. Shaved into paper-thin slivers — often misnomered in English as “bonito flakes” (bonito is a different species of fish) — this is the essential source of flavor in the dashi soup stock that underlies all Japanese cuisine.

Simple but satisfying: A bowl of freshly shaved katsuobushi on rice. | ROBBIE SWINNERTON

Nagamatsu, an energetic woman who habitually sports a smile, a bandana and a T-shirt that proclaims “Katsuo 100%,” has made it her mission to celebrate and spread the word about this traditional ingredient and the artisans who still harness the age-old methods to produce it. Katsuo Shokudo, which she opened in Shibuya in autumn 2017, is now the primary vehicle for her evangelism — and it’s every bit as unconventional as she is.

For a start, it only opens from mid-morning until around midday, operating three to five days a week depending on her other activities. She works from behind a cramped, eight-seat counter that reverts each evening to its original incarnation as a bar. And she clutters the space with books and articles about katsuobushi, and even soft toys in the shape of skipjack fish.

But her most radical move of all is that, instead of using pre-cut flakes, she shaves all the katsuobushi herself manually, using a classic “plane” set into a large box that she places front and center in her open kitchen.

She keeps four different types of katsuobushi on hand — each produced in a different way or from fish of varying provenance — giving out samples so you can taste which one you want with your meal. Then she takes a heaping mound of those aromatic shavings, and piles them up on your rice bowl until they dance and writhe from the heat of the cooked grain beneath.

Rise and shine: The breakfast set at Katsuo Shokudo, with an extra serving of dashimaki tamago omelet. | ROBBIE SWINNERTON

This forms the centerpiece of your ¥800 breakfast at Katsuo Shokudo. The rest of the meal follows the classic Japanese formula: A serving of homemade nukazuke pickles on the side; miso soup that is prepared from dashi made — of course — with the same premium katsuobushi flakes; plus (as an optional ¥150 supplement) the golden yolk of a raw egg, to mix in with your rice.

For a couple of hundred yen more, Nagamatsu also offers a slightly more deluxe version of her basic meal, supplementing your tray with a side dish of dashimaki tamago, a thick, fluffy omelet almost as soft as souffle. This is highly recommended.

Tasting is believing. Once you’ve tried premium katsuobushi freshly shaved moments before, with its deep umami savor and almost bacon-rich aroma, you may never again be satisfied with cheaper, commercial alternatives. You are likely to shun commercial instant dashi stock and banish the synthetic, one-dimensional boost of industrial white taste enhancers forever.

Even if you don’t speak much Japanese, Nagamatsu’s enthusiasm is infectious. You may even be tempted to go out and buy your own equipment, and read up about what makes traditional katsuobushi so special.

At the very least, you are likely to start planning a return visit to Katsuo Shokudo. But before you do, be sure to check the schedule, which Nagamatsu posts on her Instagram feed.

Open 9 a.m. until sold out (Sat. and Sun. from 10 a.m.); irregular closing days; set breakfast from ¥800; Japanese menu; little English spoken

Katsuo Shokudo Restaurant Page Katsuo Shokudo Name (J) かつお食堂 Lunch price range Up to ¥1,000 Cuisine Japanese Number of seats 8 Smoking No smoking Reservations Not accepted Area Shibuya Address The Neat Aoyama 1F, 1-6-4 Shibuya, Shibuya-ku, Tokyo Address (J) 東京都渋谷区渋谷1-6-4 The Neat青山 1F Stations Shibuya Station Website https://www.facebook.com/katsuoshokudou/ Hours 9 a.m. until sold out (Sat. and Sun. from 10 a.m.) Payment information Cash Map

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KEYWORDS tuna, katsuo, breakfast