Thinly slice the mushrooms and place in a vacuum pouch with the water and vacuum-pack on 100 per cent in a chamber vacuum sealer*. Be careful the bag doesn’t bubble over when doing this. Place the bag in a circulating water bath (or saucepan filled with water) heated to 70°C (158°F) and leave for 3 hours. Remove from the bath and cool to room temperature. Place in the refrigerator for at least 24 hours. This will allow the mushrooms to properly infuse the stock and create the maximum amount of flavour.
Strain the mushrooms through a fine sieve, pressing down on them with the back of a large ladle to extract as much liquid as possible. Set aside the mushrooms for another use. (They won’t have a lot of flavour but they will have a great texture. They’re a favourite addition to a staff meal at Attica of rice, pork floss and oyster sauce.) Pass the mushroom stock through muslin (cheesecloth).
Place the mushroom stock and kombu in a small saucepan, heat to 70°C (158°F) and infuse for 15 minutes. Check the stock regularly with a thermometer and ensure that the temperature doesn’t exceed 80°C (176°F). When the 15 minutes are up, remove from the heat, immediately add the bonito and infuse for 5 minutes. Pass through muslin (cheesecloth) and season with the soy sauce, salt and sugar.
Pour the stock into a 1 litre (
Return the stick blender to an upright position with the blades firmly on the base of the jug and blend the stock on the slowest setting — this will help prevent the stock ‘whirlpooling’ and drawing in air. While blending, add the xanthan gum very slowly,
Once all the xanthan has been added, readjust the seasoning, then pass through a sieve and set aside until needed.
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