It’s a Thursday morning at Katsuo Shokudo, a basement breakfast diner in the backstreets of Shibuya decorated with fish-themed paraphernalia. Today is slow and relaxed. Dressed in shades of indigo, most notably a T-shirt proclaiming “KATSUO 100%,” proprietor Mai Nagamatsu is a one-woman show in charge of the entire operation today, washing and draining rice, frying fish, taking orders, sharpening her knives, now and then breaking out with a stream of lively banter in her bright, ringing voice. The name of her restaurant translates to “Skipjack Tuna Cafeteria,” and as the name suggests, it’s all about the eponymous fish. It specializes in all things katsuobushi, or skipjack tuna flakes – the smoky, piscine backbone of Japanese cuisine.