Preheat the oven to 200°C (400°F) or 180°C (350°F) fan forced. Place the leek, carrot, ginger and onion in a large baking tin. Season and roast for 30 minutes. When cooked, pour everything from the tin into a large stockpot.
Add the dried mushrooms, garlic, spring onion and kombu. Pour in 2 litres (70 fl oz) water. Bring to the boil and then turn down to the lowest simmer. Place a lid on top and keep almost entirely closed, leaving just a small crack open. Simmer for 1 hour 30 minutes, adding the bonito fl akes in the last 5 minutes, and then strain the liquid into a smaller clean pot.
Pour in the soy and mirin, tasting to see if the stock needs additional salt or soy.