In a 4-quart heavy saucepan bring water to a boil and add bouillon cubes, stirring until dissolved.
Add beans and simmer, stirring occasionally, 15 minutes.
In a heavy skillet heat oil over moderately high heat until it just begins to smoke and stir-fry carrots until crisp-tender, about 3 minutes.
Stir carrots into soup.
In a small bowl stir together miso and 1/2 cup hot broth until combined will and stir into soup.
Bring soup just to a boil, stirring occasionally, and stir in scallions and pepper to taste.