Marbled beef from the back or belly, frozen and shaved into thin rolls that cook quickly in hot pot. Cheaper rolls are often lean beef and beef fat pressed into a frozen block.
Féiniú fàngjìn guō lǐ shuàn shíjǐ miǎo jiù shú le.
Dip the beef slices in the pot for a dozen seconds or so and they're done.
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肥牛 is not just "fat beef": it is a marbled cut, frozen and shaved paper-thin into curls, sold as 肥牛卷 for hot pot. 肥牛饭 is rice topped with simmered beef slices, much like Japanese gyudon.