Edit Environment Variables via command line

I know how to udpate my environment variables using the stupidly slow to navigate windows. This is pointlessly slow IMHO.

How do i type a command, ONCE, that will update/add an environment variable that does not need to be retyped every time i open a new CLI session. The SET command seems pathetically lacking in this ability unless it has some undocumented argument beyond the two listed with /help.