Cooking fish to perfection isn’t as easy as placing it in an oven. Start with fish quality and in Mykonos most fish is imported. The eye of the fish tells part of the story and here we find the fish has a blood shot eye which means trauma.
We except more and more the quality available and those fishing habits of so-called professionals who abuse their catch and really have no idea how to treat fish after they are caught deliver damaged goods.
see ike-jime: https://mesubim.com/2013/02/27/ike-jime/
So stick with the cooking, and the professionals have their ways and if you decide to cook fish at home invest in some tools, such as a fluke thermocouple.
I start with placing a needle into the spine at the top side to measure the temperature.
Now depending on the fish, set the oven at 180°C and use steam and heat. Now wait until you reach 62°C and remove the fish and rest it. The fish’s internal temperature will rise by 2-3°C and finishing temperature is 65°C.
PS: when cooking John dory it is a different density and the fish isn’t nearly as thick as sea bass, so it may require cooking with a finihsing temperatire of 65°C on the thermocouple.