Add water, bring to a boil over high heat, then lower heat and simmer covered until carrots are tender, about 10 minutes. Remove from heat.
Place miso in a bowl, add a little of the broth from the soup, and stir into a smooth paste. Add more broth to thin the mixture, then add the miso to the soup. Let rest for a few minutes. (Miso should never be boiled.)
Serve in bowls with chopped raw green onions and a few drops of roasted sesame oil.
Recipe by forkbelly.com at http://forkbelly.com/miso-soup/