For the soup, shave the cucumbers and keep the shavings for later.
In a blender or liquidiser, blend the cucumbers (not the shavings), the ginger, vinegar, lime juice, miso and vegetable stock until smooth, then add the spring onion greens and spinach and continue to purée until smooth. Pass through a fine sieve into a bowl and leave to cool in the fridge.
For the salmon, heat half of the oil in a frying pan, season the salmon with salt and fry in a hot pan for 30 seconds on each side. Remove from the pan and set aside to cool, then cut into 5mm/¼in dice. In a bowl, combine the fish with the remaining ingredients for the salmon. Cover and leave to marinate for 10 minutes.
Finely chop the whites of the spring onions.
To serve, pour the soup into four chilled bowls and garnish with the salmon, cucumber shavings and shredded spring onion.