How can I recover a draft of an email I accidentally deleted.

If the draft was there long enough for Time machine to back up you may be able to get the draft box back as follows:

To restore mailbox:

Open Finder

Click “GO” in top menu bar then click option key

Click on Library

Open mail folder

Open V9 or V10 (or similar) folder

Then find the folders labeled with cryptic titles (at this point you could open TM if you want to go back in time)

Look down through those folders to find a mailbox (note: there may be more than one with same label).

If it contains email you want you can copy the entire folder to your desktop

Go back into mail and import that folder from your desktop

I doubt you can. Draft's by definition are "temporary" and once gone it's gone.