IOW, the classical scientific method. I submit that this is missing from science education, too. And likely from math education, as well, with the demise of the rigorous proof-heavy geometry class.
Bingo.
A lot of time, money, and wasted effort could be saved by doing that ONE thing well. It's not the facts or even particular micro-skills that are important. After all, it's not often that I've been asked to dissect a bovine eye in my adult life, after all, or to identify a precipitate based upon color alone, but actually teaching students the rigorous mental discipline necessary, and the logic inherent in the scientific method-- and in mathematical proofs-- now
that is education I'd like to see. For everyone.

Teach people that, and IMO you have given them the TOOLS to be true autodidacts through their lives-- and to
learn anything in STEM. At will.