Soak the walnuts in cold water overnight in the refrigerator. The next day, place the jug of an upright blender in the freezer for 30 minutes. Drain the walnuts and peel off the skins. Chill the walnuts to 4°C (39°F).
Put the walnuts, garlic, iced water and a pinch of salt into the blender jug and blend on high speed until a smooth consistency. Continue to blend until the purée begins to warm due to the friction of blending. Pass the purée through a fine sieve and place in the freezer. Clean the blender jug and place in the freezer again. Chill both for 30 minutes.
Place the purée back in the blender jug and, while blending, slowly add the oil, then the lemon juice, and cider and balsamic vinegars. Continue blending for a further 5 minutes or until the purée is silky in consistency. Adjust the seasoning with salt, then pass through a fine sieve. Pour into an airtight container and chill in the refrigerator. Once chilled, re-check seasoning. This makes more than you will need.
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