How do you send mail to Undisclosed Recipients? Not something I really ever need to do often, but just how do you send to Undisclosed Recipients in Mail.app  

I've seen them arrive looking like this -

but never figured out how I might do it myself

Previously, I've sent to myself, then everybody else as BCC, but is that really the way to do it?
That's the way to do it, yes. You can even leave the To: part empty which then will result in Undisclosed recipients:.