Begin by soaking the kombu the night before. Place the kombu in a pan with the soft mineral water. Cover, place in the fridge and leave overnight
The next day, place the pan over a medium heat and gently heat until it reaches 65-70ºC. At this temperature, remove the kombu and continue to heat the water until the temperature reaches 85ºC
As soon as the temperature hits 85ºC, turn off the heat and stir in the bonito flakes. Leave to infuse for 1 minute
Strain the stock through a colander and then again through muslin cloth into a container – it should be a beautiful clear golden colour. Use right away or store in the fridge until needed