I think the approach posted by albsa will work well. Of course, even better would probably be to use the new api-routes, so I second your call for a tutorial!
thank you very much for that link! I wrapped my head around it and I really dislike the available solutions. It adds plenty of overhead for simple authentification by adding an express server to handle routing.
I am going to implement my own solution without the need of a server using getInitialProps(). This approach should be way more maintainable.
Just to add my vote to having an updated approach to the original tutorial! @Niksn totally agree with you. Would be grate to have a look to your solution when you have it!