Combine the sweet potato with the broth or water with bouillon cubes. Bring to a gentle boil, then lower the heat and simmer gently until the sweet potatoes are tender, about 15 minutes
Add an additional cup of water, then stir the tofu in gently, followed by the scallions and ginger.
Return to a rapid simmer, then remove from the heat.
Dissolve 2 tablespoons of miso in a small amount of warm water. Add to the soup and taste. If you’d like a saltier, more pungent taste, add another tablespoon or two of miso dissolved in a little warm water.
Season with pepper and stir in most of the greens, saving a small amount to top individual portions.
Serve at once, topping each bowlful with a handful of uncooked greens.