Mai Nagamatsu is more than just a fan of katsuo (skipjack tuna). She's an avowed, unapologetic evangelist for those streamlined, silvery pelagic fish that play such an important role in Japanese cuisine. So much so that she set up her own small specialist breakfast counter to help spread the word.

Initially, her Katsuo Shokudo ("Skipjack Diner") operated as an irregular pop-up, taking over a small bar in Shibuya for morning sessions. But last summer she moved into her own dedicated premises in a bright, spacious basement a seven-minute walk from the other side of Shibuya Station.

While Nagamatsu loves skipjack in all its forms, her primary focus is on katsuobushi, the dried, cured fillets as hard as tropical timber, which she planes by hand into shavings, to add their deep umami to soups and other dishes.

Now she can seat 13 customers at a time for each one-hour slot, serving her simple, nutritious set meals of rice topped with freshly planed katsuobushi flakes, along with a side dish, pickles and miso soup. She also opens for occasional evening sessions, which will eventually become more frequent. For the full schedule and advance bookings — essential most days — check the Katsuo Shokudo Instagram feed.

GranDuo Shibuya B1F, Uguisudanicho 7-12, Shibuya-ku, Tokyo 150-0032; 050-3267-8669; katsuosha.stores.jp, instagram.com/katsuoshokudou; open 9 a.m.-2 p.m. and occasional evenings; irregular closing days (listed on Instagram); set meals from ¥1,100; nearest station Shibuya; nonsmoking; cash only; Japanese menu; English not spoken