Encrypting a mounted APFS drive

Pre-Mojave: It used to be if you had an external HDD or SSD (not the boot drive) that Finder would allow you to do an "encrypt-in-place" on it in the background.

Mojave and later: Finder allows me to *decrypt* a mounted & encrypted APFS or Mac OS Extended drive, but the option to go the other way has been removed.

Is there a way to encrypt my APFS-formatted external drive without losing the contents?

-Carl