It was only today I learned some cuts of beef are not kosher in the US due to the sciatic nerve and some "forbidden fats"?

I stumbled across a discussion of the cost of flank vs skirt which led to my finding out skirt steak is kosher but flank is not. I’d never heard that not all cuts of beef were kosher before.

Uh yeah, I knew about split hooves and chewing cud (which does allow for giraffes, and wildebeests)

That led to a discussion of Jacob and the sciatic nerve, which I sort of get. But this Forward article says there are “forbidden fats” too.

What are the forbidden fats and what made them forbidden?

And regarding both the forbidden fats and the sciatic nerve (and it’s removal), is that Torah law or Rabbinical law? If Torah, where and can you help me understand the text? If Rabbinical by whom and when?

Also, please feel free to post your favorite Romanian steak recipe