To make dashi, put the water and kombu into a large saucepan and bring to 90°C (195°F). Remove from the heat, immediately add the katsuobushi and allow to infuse for 1 hour. Add the soy sauce and mirin to season. Strain the dashi and discard the solids.
Roast the blanched almonds in a medium oven until very lightly toasted. Warm the dashi in a small saucepan. Put the dashi and almonds into a blender and process for 2 minutes on high. Crush the citron leaves in your hand and add to the blender. Blitz for 5 seconds. Remove the mixture from the blender and allow to infuse in a bowl for 20 minutes.
Pass the mixture through a fine muslin-lined sieve, applying a small amount of pressure to assist the extraction. Discard the solids. Put the liquid into a fresh saucepan. Whisk in the kuzu starch until smooth, then put the saucepan over medium heat as you constantly whisk. The mixture will rapidly thicken. Turn the heat to low and continue to whisk, cooking out the starch flavour, for a minimum of 5 minutes. Put the thickened mixture into a small plastic container, cover and refrigerate. This mixture needs to be made a minimum of 6 hours before assembling the finished recipe, and can be made up to 3 days in advance.
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