Place the mushrooms in a vacuum pouch with the water and vacuum-pack on 100 per cent in a chamber vacuum sealer*. Place the bag in a circulating water bath (or a saucepan filled with water) heated to 70°C (158°F) and leave for 3 hours. Remove from the bath and cool to room temperature. Refrigerate overnight or for at least 12 hours. This will allow the mushrooms to infuse properly and create a full-favoured stock.
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. 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.