Mexicans would be horrified. Even the Japanese might need a moment. But here it is: a newly opened sushi taco spot near Jing'an Temple.
It's a small street-side shop doing oversized hand-held sushi... things. Somewhere between a taco and a poke bowl, each one is palm-sized, seaweed-wrapped, and stuffed to the edges—crab stick, grilled eel, salmon, that sort of thing. About ten flavors in total.
Most go for around 10rmb. You could get full for under 20. Eel and salmon get the most love online.
Xiao Ta Zong Taco Zone! @216 Wanhangdu Road, Jing'an District