These little mushrooms appeared overnight in my garden (located in the Mediterranean region). I was wondering if anyone could help identify what they are. Closest species I got to was Mycena pelianthina but I'm almost certain it isn't it. Stalk is about 5mm wide, 3-5cm long, cap about 3.5cm wide. Gills barely apparent and are the same colour as the cap.
You are right when you say you are wrong. This is nothing like Pelianthina.
This is clearly a close member of the Coprinus Genus. The hard part is those guys are dozens of sorts (and the genus got deeply reorganized recently in 2001)
But that one is pretty common I think, very similar to the one I had here (France).
So I would say this is Coprinopsis Lagopus, because of the quickness of its growth and the way the cap developed:
By Tim Sage - This image is Image Number 122149 at Mushroom Observer, a source for mycological images. This tag does not indicate the copyright status of the attached work. A normal copyright tag is still required. See Commons:Licensing for more information., CC BY-SA 3.0, Link
It also could be Coprinopsis Cinerea. Difficult to say.