Is it possible to recover a deleted APFS volume? Nothing’s been changed or formatted. The “container” is not mounted and the volume I accidentally deleted still shows up in the Boot menu at startup. 

How would I go about recovering this APFS volume?
Most commercial file recovery utilities for the Mac also will perform recovery of deleted partitions. Disk Drill Pro, EaseUS, Stellar Data Recovery Pro, etc.

Primary concern is avoiding writing anything to the drive with the deleted partition. Don't run any disk repair/maintenance utilities or attempt to re-create the partition. Avoid using the affected drive (and if it's the boot drive of the Mac, then the Mac itself) at all if possible. My suggestion would be either:


boot the Mac into Target Disk Mode and run recovery software from a different Thunderbolt-connected Mac
(if your recovery software offers this option) create a bootable recovery USB drive on a different Mac and boot the affected Mac from it