Tails from a fishmonger, ideally skinned with the grey-purple membrane removed — ask them to. The cheeks are a treat if you can get them. Rape in Spain, lotte in France.
Worth knowing
Peel off every bit of membrane, or it shrinks and curls the flesh. It's watery: salt it for 20 minutes and pat dry before roasting, and take it off at about 55°C — it goes from moist to rubbery fast. Roast the bone for stock.