“What do you think this is?” asks one of the masters behind the counter. You take the shirako nigiri in your mouth. The sea-broth-flavored custard tastes unlike any part of fish you can name. After you swallow, you’re told it’s a cod organ. Later you Google shirako, only to discover that cod sperm is an absolutely delicious delicacy. Osaka-born sushi master Yasuhisa Ouchi tells each piece of fish he cuts to “taste good” before he presses the flesh into a much-fussed-over round of rice. Fans of the documentary Jiro Dreams of Sushi will be delighted by the omakase at this 16-seat Annex restaurant, where diners are treated to the whims of the chefs. The 2-hour affair includes some familiar seafood delights (fatty tuna, mackerel, Hokkaido scallop) but often ventures into esoteric territory. Thumb-sized firefly squid, sakura shrimp that actually look like cherry-blossom petals, and ankimo (monkfish liver) are seasonal highlights. This is not sushi for California-roll junkies.