Put the soup stock in a pan over a medium heat without a lid on. When it starts to simmer, add the wakame seaweed (if instructed by the package, soak it in cold water for a few minutes and drain well before adding) and tofu.
Mix the miso with the soup stock in a ladle with chopsticks. Just before it boils, add it into the rest of the soup stock.
Once it is dissolved, add the salad onions, then turn the heat off.