Reads like a middle part of the post... What exactly does author mean by "onion layering" and what are they being opposed to?

The author supplies some examples (new API field, new microservice dependency, new library dependency) but their fixes seem to apply to any well-organized code - I don't see why you'd need to use some mysterious "onion layering" to create a wrapper around insane dependency and isolate it from the rest of the code.

(Google searches show only a few results and they all seem slightly different, so I am guessing this variant of "onion layering" is something author made up but didn't share with anyone - their blog has only one post)