Simon Cowell & Ex-Fiancee Mezhgan Hussainy -- The Fabulous 'Parting Gift' Mansion

Here it is ... the $8 million "parting gift" Simon Cowell gave to his ex-fiancee Mezhgan Hussainy when he dumped her ass earlier this year -- his old, used mansion in Beverly Hills.

TMZ has learned ... Simon moved out of the home a few years ago ... when he upgraded to a spiffier 6-bedroom $12 million dream home ... just a few miles away (below).

As we previously reported, Simon felt Mezhgan "deserved" the old property for having to deal with all of the public scrutiny that comes with dating the "X Factor" honcho.

The real question -- why did Simon have an old mansion just lying around? If you have to ask, you can't afford it.