Is this fish dish actually Greek in origin?
I found an article (Polish) that may shed some light onto the origins of this mysterious dish. The first two paragraphs in translation:

"If you were to show the Greeks fried hake or pollock filets, covered with grated carrots, braised until soft with onions and tomato concentrate, asking if they could recognize it as a Greek dish, they'd probably nod their heads in an affirmative manner. At such time it's good to know that this gesture has the opposite meaning in Greece as compared to us.

"Most likely, the original recipe was psari plaki - fish baked in a tray, covered in tomato sauce with vegetables. The vegetables are prepared separately. Onions and garlic are sauteed with olive oil, sometimes with the addition of carrots and celery, perhaps some olives, a spoonful of honey and always - a lot of tomatoes. This is braised briefly with the addition of white wine. The sauce is delicate, fragrant with the wine, herbs and full of vegetable chunks. The fish is gutted, rolled in olive oil, salt, pepper and lemon juice and prepared for baking by covering it with the vegetable sauce. It should not by any means be fileted, as it is detrimental to the flavour."

So as was to be expected, our national variation is probably at most a bastardized version of a Greek dish. But who cares, it still tastes great. ;) I just made my first batch and it was delicious, I highly recommend it if you have the opportunity. :)