Cut the carrot into small cubes or thin slices.
In a saucepan, bring the water and stock cube to a boil, then reduce to a gentle simmer.
Add the bok choy and carrot, and cook for about 4 minutes until just tender.
Add the spring onions and wakame, and cook for another minute.
Turn off the heat.
Take a ladleful of the hot stock and mix it in a small bowl with the miso paste, soy sauce, and rice vinegar until smooth.
Pour this mixture back into the soup and stir gently.
Add the cubed tempeh and let it warm through in the hot broth.
Taste and adjust if needed.
Divide cooked soba noodles between bowls, then ladle over the soup. Finish with a sprinkle of sesame seeds.