Gut and clean a fish, larger fish cut into 2 or 3 parts.
In a large pan, heat 1/4 cup olive oil, coat every piece of fish in flour and fry for 1-2 minutes, until browned on every side. Set aside and keep warm.
In a large round pot, add remaining olive oil, add chopped onions and saute until translucent, add minced garlic, and saute until fragrant, around 1 minute.
Add shrimps, clams, crabs, squid or whatever seafood you are using, fry for 1-2 minutes, then add tomatoes, tomato paste, bay leaves, rosemary spring, and lemon slices. Season with salt and pepper.
Carefully add fish, pour red wine vinegar, red wine, and water just to cover a fish ( add more water if the fish is not fully covered) and cook around 30 minutes on medium-low heat.
Don't cover the pot and DO NOT STIR with any kind of ladle (as you will break the fish), just shake the whole pot (if you have wooden flat ladle, you may move fish a bit).
If necessary add more salt. Try it: if it's too sour, add sugar.
Sprinkle with chopped parsley. Carefully take the fish out of the pot and serve warm with cooked polenta.