Bring 6 cups of water to a boil in a medium saucepan. Turn the heat to low, then mix about 1/2 cup of the water with the miso in a bowl or blender; whisk or blend until smooth. (If you have an immersion blender, the fastest and easiest tool here, carry out this operation in a tall measuring cup.)
Pour the miso mixture back into the hot water and add the tofu and carrot; stir once or twice and let sit for a minute, just long enough to heat the tofu through. Add the scallion and serve.
