Pat dry the fish and season with salt.
Marinate at room temperature for 15 minutes.
Heat a good splash of olive oil to fully coat the bottom of a non-stick frying pan over high heat.
Coat the fish in flour, then shake off excess flour.
Add the fish to the sizzling hot oil. Flip after 2-3 minutes using two forks. If the oil becomes too hot (smoky hot), drop the temperature down a bit to medium-high.
Pan-fry the fish for about 6-7 minutes overall, flipping as necessary but not too often. Allow to form a nice golden crust on each side.
Transfer the fish to paper towels. And drop the heat to low.
Add onion and garlic to the pan and saute until softened.
Add rosemary and raisins and cook for a minute more.
Pour in the vinegar and cook over low heat for 15-20 minutes until a thin sauce remains. At this point, you can stir in the sweetener (sugar, honey, grape molasses) if you're using any.
Transfer the fish to a serving plate.
Serve! Add spoonfuls of the sauce over the fish. You needn't add it all, you can serve some on the side as well.