Great deep dive. I have never thought about onions and closures in the same sentence before :) Here is my attempt at explaining closures to beginners:

Learning JavaScript Closures through the Laws of Karma

I often use “Russian nesting doll” as a metaphor to explain nested scopes.

I left one minor private note for you, more to do with MDN documentation than this post. One of the confusions for beginners is around attributing closures to a tangible thing like object, function, etc… So I often find it easier to explicitly callout that closure is not a tangible thing: it’s neither an object nor a function.