Heat oil in large saucepan or pot. Add onion, carrots, and garlic, and saute for 10 minutes, until onion is softened and translucent. Add ginger and water and bring to a boil. Reduce heat, cover, and simmer for about 30 minutes, until carrots are tender.
Remove from heat and puree with an immersion blender (or use a regular blender). Add miso to a bowl and ladle ½ cup of soup into it. Whisk until miso is dissolved. Add miso mixture to the soup and stir well. Add pepper and salt if desired. Top with scallions and a tiny bit of sesame oil.