20 lamb kidneys, cut in half
90 ml (3 fl oz) vinegar
- Put the kidneys in a glass bowl, cover with cold water, add the vinegar and leave for 30 minutes.
- Meanwhile, heat the olive oil and butter in a large frying pan. Add the onion and fennel, season well and cook over very low heat without browning for 20–25 minutes. Transfer to a bowl.
- Core the kidneys and thinly slice. Add to a dry frying