Heat the oil and wine or stock in a deep pan, then add the celery, garlic and onions and fry for 5-6 minutes, stirring frequently, until they have softened.
Roughly chop the tomatoes (they can be peeled if you like) and add them to the casserole with the lemon, tomato purée and the sugar, if needed (it depends how acid the tomatoes are). Bring to the boil and simmer, stirring, for 2-3 minutes until the tomatoes begin to soften.
Arrange the fish in a single layer on top of the vegetable mixture. Tuck in the sprigs of rosemary, sprinkle with oregano and season to taste.
Cover the casserole and leave it to simmer for 10-15 minutes until the flesh is just firm, spooning the juices over the fish occasionally. Serve garnished with chopped oregano and a sprig of rosemary.
the olive oil can be omitted, if you prefer, but even a little helps to capture the Mediterranean flavour.