Place the currants in a bowl and cover with freshly boiled water. Leave for 15–20 minutes, then drain thoroughly.
Toast the pine nuts in a dry frying pan over medium-low heat for 2–4 minutes until lightly golden. Set aside.
Pat the sardine fillets dry with paper towel and season lightly with sea salt.
Spread the flour over a plate or tray and lightly dust the sardines, shaking off any excess.
Heat the olive oil in a large frying pan over medium heat.
Working in batches, fry the sardines skin side down first for 1–2 minutes until lightly golden. Carefully flip and cook for another 30 seconds to 1 minute until just cooked through.
Transfer to a paper towel-lined plate and sprinkle lightly with a little extra sea salt while still hot if needed.
Carefully pour off most of the oil, leaving about 2–3 tablespoons in the pan.
Reduce heat to medium-low and add the onions, bay leaf and a pinch of salt.
Cook for about 25 minutes, stirring regularly, until the onions are soft, lightly coloured and jammy.
Add the drained currants, caster sugar and white wine vinegar.
Simmer for 2–4 minutes until the sugar dissolves and the vinegar smells sharp.
Stir through the toasted pine nuts.
Use a shallow storage container or dish and spoon a thin layer of onion mixture onto the base.
Arrange a layer of sardines over the top, then add more onion mixture.
Repeat until everything is used.
Cool completely, cover and refrigerate for at least 4 hours or overnight.
Remove from the fridge 45–60 minutes before serving and transfer carefully to a serving platter.
Serve at room temperature.