To prepare broth: Place the dashi stock and white miso together in a pot. Heat the stock and stir until the miso dissolves. Bring to the boil. Add the chopped salmon fish head, onion, leek, white cabbage and cherry tomatoes. Simmer over low heat for about 10 minutes. Remove from heat.
To serve: Place the cooked soba noodles in a serving bowl. Top with the spinach, wakame, spring onion and naruto maki. Add the salmon fish head, onion, leek, white cabbage and cherry tomatoes from the soup broth. Ladle hot soup broth into the bowl and serve immediately.