Peel back the onions to the young flesh. Put the onions in a vacuum bag with the vinegar, oil and salt. Seal and cook in a water bath at 78°C (172°F) for 35 minutes. Plunge the bag into iced water and leave until chilled.
When ready to serve, pour the onions and any juices from the bag into a saucepan and warm through. Keep the onion juices for warming the wild fruits, right.
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