加熟. make it well-done.

That means they want to recook the steak until it well-down.

Thanks, my friend, you are very helpful :)

You are welcome. I think that's a very good question. ^ ^ the endings .with "分熟" is correct. '成熟" is never used to express how the food is cooked in such case. I am not surprised with the variants you found. The number before 分 is mostly given by personal standard, regarding "十分熟” as well-done and 一分熟 as raw. 熟 here is pronounced as "shóu". And actually in most spoken Mandarin, it is pronounced this way. Only in written form or very formal occassion, it may be pronounced as "shú".

Wow, thank you, you actually reminded me about one thing I encountered yesterday while checking the ways of saying these "medium, medium-well". Some resources give the endings ...分熟，and some 。。。成熟. Which one is more correct? Or are both okay? Then there was a mess in variants, I couldn't figure out which one was "rare" and which one "medium rare", etc. Just googling wasn't of much help, but you really placed them all in order, thank you very much!!! :) One more little question: this 熟 is pronounced as "shu" or "shou"?