Rinse the sardines under cold water then pat dry with kitchen paper. Place the flour in a large bowl, add the sardines and toss to dust evenly. Shake off any excess
Fill two-thirds of a medium, high-sided frying pan with sunflower oil. Place it over a medium-high heat and wait until it reaches 180°C
Slip in a batch of sardines (6–8, depending on size). Fry for a couple of minutes on each side, or until crisp outside and opaque inside. Drain and transfer to a plate covered with kitchen paper. Salt generously and repeat with the remaining sardines, working in batches
Discard the oil with the exception of a very thin film – just enough to coat the bottom of the frying pan. Set this back over a medium heat, add the onions and sweat gently until very soft, stirring often so they don’t brown
Next, whisk the vinegar with the sugar and pour it in. Reduce the liquid to about half its volume, remove from the heat, taste and season with salt and pepper
Arrange a layer of sardines in a large glass bowl. Cover with a layer of white onions and then repeat in layers until you have finished all the ingredients, finishing with onions. Pour any remaining cooking liquid on top. Leave to cool to room temperature before wrapping in cling film and setting in the fridge to marinate for 24 hours
