Or, a more simple way.
After crash, after you've started Opera and encountered an empty new session, while Opera is open, do a back-up of this folder -> %AppData%\Opera Software\Opera Stable\Sessions
Then you can close Opera, rename/delete current "Sessions" folder, and replace it the one from backup. Keep the backup until you will have a recovered session. Make sure in current "Sessions" folder is only by one "Session_####" and "Tabs_####" file (the smallest one can be deleted).

Important: When you notice that your session was lost, do not start browsing in this new empty session to not overwrite your old main session. For browsing Internet use another browser, another device, or a private windows in Opera (Ctrl+Shift+N).