I bought it very cheap during the closing of the grocery market. The seller said that it was some kind of lettuce, but it tastes like upper leaves of cabbage.
It appears to be curly endive which is a lettuce. It is crisp and has a bitter flavor. Dole Know Your Lettuce
I used to work in produce and at first glance, I thought this was a Bok Choy Cabbage, but I also believe it's the Curly Endive Lettuce.