/diy/ - Do It Yourself

How can I open up and clean the inside of this sink? (pic related)

There's no screw in the center - instead the plug goes in there.

It's draining slow and shitty, I've removed, cleaned, and reassembled the drain pipes, but that hasn't helped - you can even see muck inside in the picture.

Drain cleaner didn't do shit (don't know why I even hoped it would).

So how do I disassemble this without breaking anything?

(It's a rental flat in another country - I'm not a plumbsmen on the best of days, I do woodworking and homebrewing and such)