How do you onboard to a new framework?

If is immediately clear how the problem fits the model conceptually, if the framework doesn't require deep new skills, and the tooling, documentation, such are good, the answer is "start building".

(for instance, imagine some Sinatra-like web framework which could be in any programming language -- the deep complexity is in the nature of http for which your past experience is portable...)

If the above are missing you look to remediations such as: maybe reading the source code is better than the reading to docs, intensive training in the subject (i challenged myself to make a print a day and now I have a pile of burned out thermal printers, samples of every industrial adhesive I could find, and walls covered with photo prints and anime art), doing a pilot project to scout risks (e.g. build an instance bigger than prod will be on day four), etc.

Also: frequently the "onboarding" is to a system that already uses the framework. For instance, this is how React found me. Then thinking hard about the problem of "how do I update the UI when I receive data from the server?" did pilot projects in Vue and MobX-React.