Preheat the oven to 200°C/fan 180°C/gas mark 6. Soak the raisins in enough boiling water to just cover them. Set aside for the salad.
Begin the stuffing by dry-toasting the pine nuts over a low to medium heat in a small heavy-based frying pan until evenly coloured. This will take just a couple of minutes.
While the pine nuts are toasting, butterfly the sardines and discard the backbone. To do this, open up the fish cavity and run your thumb down the centre of the spine to loosen it. Pull the spine out gently, from the head end, taking care not to tear the skin. Open each sardine out flat.
Finish off the stuffing by mixing the toasted pine nuts with the breadcrumbs, feta, parsley, garlic and lemon zest. Season to taste with salt and pepper. Spoon the stuffing evenly in the cavity down the middle of each sardine. Fold the opened-out sardine flesh back over like an envelope to enclose the stuffing. Tie with butcher’s string to secure. Lay the sardines on a baking sheet lined with greaseproof paper, then bake in the preheated oven for 8–10 minutes.
Meanwhile, make the salad. Pour boiling water over the sliced red onion to soften it. Leave for a few minutes and then drain off the water. Drain the soaked raisins and mix them with the parsley leaves, capers and drained onion in a bowl. Season to taste with salt and pepper, dress with the extra virgin olive oil and sherry vinegar and serve alongside the sardines.