Place shiitake mushrooms into a heatproof bowl. Cover with 1 litre + 200mls of just boiled water and cover bowl with plastic wrap. Soak for 30 minutes.
Meanwhile, peel carrots then use a mandoline to finely slice both the zucchinis and carrots (if you don’t have a mandoline, just use a sharp knife to slice as finely as possible). Add oil to medium sized saucepan over medium heat then add ginger, garlic, chilli flakes, carrot and zucchini. Sauté slowly, covered, for a few minutes, stirring regularly until garlic is transparent yet the zucchini and carrot are still crisp.
Remove vegetables from the saucepan onto a plate and set aside.
At the end of soaking time for the mushrooms, place a sieve over the saucepan and drain the mushrooms, reserving the mushroom water into the saucepan.
Finely chop the drained shiitake mushrooms and set aside.
Add a tablespoon of miso paste and chopped shiitake mushrooms to each bowl.
Bring mushroom water to the boil in the saucepan and place in soba noodles and simmer on medium heat for 3 minutes, or until tender.
Pour the soup and noodles evenly over the miso paste and shiitake mushrooms in each bowl and whisk until the miso paste has dissolved. Add the vegetables, spring onions and baby spinach to each bowl and immerse in the liquid so the spinach wilts.