10 salted herring fillets (about 1.3kg/3lb) rinsed, soaked in water overnight, drained
red onion, finely chopped, to garnish
handful chives, finely chopped chives, to garnish
For the caper butter sauce
110g/4oz unsalted butter
1 tbsp finely chopped dill
½ lemon, juice only
salt and freshly ground black pepper
8 slice rye bread, buttered
Put the vinegar, sugar, bay, carrot, onion, peppercorns and mustard seeds in a saucepan and bring to the boil over a medium–high heat. Reduce the heat and simmer for 5 minutes, or until the sugar has dissolved. Remove from the heat and leave to cool to room temperature.
Put the herring fillets in a non-reactive bowl. Pour over the cooled pickling liquid and stir. Cover and marinate in the fridge for 2 days.
To make the caper butter sauce, melt the butter in a frying pan. When it is turning brown, add the capers and cook for 2 minutes. Add the dill and lemon juice. Season with salt and pepper.
To serve, remove fillets from the pickling liquid and pat dry using kitchen paper. Cut them into 7.5cm/3in pieces and transfer to the buttered rye bread slices. Pour over the caper butter sauce and garnish with the red onion and chives.