Peel and trim the onion. Slice it up finely and transfer to a saucepan. Pour in the vinegar and water.

Place it over medium-high heat until boiling. Then let the vinegar reduce at least by half. Season with a little pepper and salt to taste. Keep the onions crunchy!

Then remove the pan from the heat. Stir in the raisins. Let the onions rest until cooled down.

In the meantime clean the fresh sardines thoroughly. Rinse them under cold running tap water. Pat them dry with some kitchen paper. Pour the plain flour on a plate. Season with a dash of pepper and salt. Then lightly coat the cleaned sardines with the flour. Shake off any excess flour.

Pour a little olive oil in a non-stick pan and place it over medium-high heat until hot. Add the floured sardines skin side down.

Brown the sardines in the hot oil on both sides for 2 minutes. They should be golden and crunchy.

Then take the pan off the heat. Put a layer of vinegary onions in a baking dish. Top with half of the sardines.

Cover with another layer of onions and sardines again. Finish with the remaining onions.

Let the sardines rest at room temperature for at least 5 hours until fully marinated. Serve cold.