Yet another (big) oil leak - what to do?

[tudorv]

Hi guys. I was riding today and during a stop I notices the engine block was dirty and oily. So I wiped it off and kept riding and saw oil bleeding out through that gasket. I happens mostly at high speeds, when the engine is in high rpm. It's a pretty big leak as it messed up my pants and had oil on the bike all the way to the end of the exhaust.
I tried to tighten that bolt, but it didn't make much of a difference. Rode it slow all the way home and kept an eye on the oil level.

So my question is: how bad is this? Do I just need to replace the gasket? Or it's indicative of a bigger problem? Could this have been caused by the fact that my last (stupid) mechanic put a little bit more oil in the bike, over the full limit?

Please give me some advice if you can. Thank you. Here's the picture of the thing. The oil leak is exactly where the arrow is pointing: