What is the best way / learning plan to learn Eliom/Ocsigen?
I think the official tutorials are quite thorough covering many topics and you can start there.
Indeed the tutorial is the best starting point.
I would recommend the tutorial about Ocsigen Start, which is the best way to have quickly a fully working app with code examples. (Best way to start without reading a lot of documentation).
Start with the tutorial, then if you are stuck while working on a project (and this applies to pretty much any other hefty library/framework/toolkit for ocaml) and you can’t make out what to do from reading the docs, code search on github for specific examples.