Boquerones en vinagre

Method

1. Rinse 150 g of very fresh anchovies under cold running water. Pinch off each head and pull it away with the guts attached, then run your thumb along the belly to open the fish flat.
2. Lay each fish skin-side down and lift the backbone out from the head end, snapping it off at the tail so you are left with two joined fillets. Drop the fillets into a bowl of iced water and swirl for 10 minutes, changing the water twice, until it stays clear and the flesh is pearly white with no blood.
3. Drain and pat the fillets completely dry on kitchen paper. Pack them flat in a sealed container and freeze at -18C or colder for 48 hours — this step is not optional, it kills anisakis parasites, and vinegar alone will not. Thaw overnight in the fridge before continuing.
4. Arrange the thawed fillets skin-side down in a single overlapping layer in a shallow non-reactive dish. Scatter 1 tsp of fine sea salt evenly over them, then pour over 100 ml of white wine vinegar mixed with 50 ml of cold water, enough to submerge the fish.