First, very carefully debone the fish fillets with kitchen tweezers and cut them into small cubes. Soak the dried mushrooms in lukewarm water for 20-30 minutes. Then lift out the mushrooms, rinse with cold water and remove hard stems. Cut larger mushrooms into smaller pieces. Cut tofu into bite-sized cubes. Wash spinach leaves, drain and coarsely chop. Now in a large pot, bring soup to a boil. Add mushrooms and cook briefly. Then add the tofu and fish cubes and cook for about 10 minutes. Once the fish is cooked, stir in the spinach leaves and cook briefly. Season with salt, pepper and sesame oil before serving.