Rinse the fish and pat dry with paper towels. Season with salt and pepper.  Heat 1.25cm (1/2in) oil in a wide, shallow frying pan over a medium-high heat.  Dredge the fish lightly in seasoned flour and shake off the excess. When the oil is hot lay the fish carefully into the pan. Fry in batches until golden and crisp on one side, 2–3 minutes, or longer depending on the thickness of the fish.

Gently flip with a fish slice and cook on the other side for a further 2 minutes.  Remove the fillets from the pan with a wire skimmer and drain on paper towels.  Lay the fried fish in a single layer in a large heatproof serving dish.

Make the marinade: When the fish is cooked, pour off the oil and wipe the pan clean. Return the pan to the stove. Heat the olive oil, add the garlic and stir for 1 minute. Stir in the remaining marinade ingredients.  Pour the heated marinade evenly over the fish and leave to cool completely.

Cover with plastic wrap and refrigerate for 2–3 hours or overnight. Bring the fish up to room temperature when ready to eat.