Bring stock to boil. Reduce heat to minimum. Steam zucchini and turnips for 2-3 minutes. Set aside. Mix the shiro miso and tahini with 1 ladle of stock and add to the soup. Mix well.
Using a hand blender or blender, blend the soup at maximum speed to emulsify the liquid and obtain a cappuccino effect. Divide the broth between bowls or soup plates. Add rice, zucchini, turnips, avocado and lemon zest. Sprinkle with nori chips.
Tip: Add the shiro miso at the end and off the heat, otherwise it may curdle. Toast the nori sheets in a non-stick pan or grill for 1 minute on each side. Leave to cool. Break into chips.