What Did Basara Do to Mio to relieve the curse?
The Scene you want to see is showed in the restaurant scene as flashback while Basara and his friend is talking about the Master- Servant Pact.

Are You sure you have the Uncensored version. I watched the anime and here is the screenshot of scene flashback from the episode following the restaurant scene.





Also there is also a special Episode, ShinMai Maouno TEstament Special 3 one of the Specials included on the Blu-ray/DVD volumes. Chibi versions of Mio and Maria narrate Basara's various erotic encounters.



Here is a Screenshot of the Third Special Episode (Narration of Episode 5's Erotic Event)

ALso Zhi Heng's Observation and answer is quite Correct. Basara did what he have to do to stop the activation of Mio's curse.