Update - the mechanic said there is a pressure-release valve on the turbo engine that sometimes sticks and the resulting back-pressure causes oil to blow through the weakest point - in this case, the oil filler cap.

They replaced it (under warranty) in about 30 minutes.

 

Specifically, it was the crankcase ventilation system pressure regulating valve.

So there you have it.

 

well thats good, at least you got the problem solved without having to pay anything