Combine the stock with the scallions and kombu, if using, in a soup pot. Bring to a simmer, then cover and simmer gently for 5 minutes. Stir in the tofu. Simmer 1 to 2 minutes longer, then remove the kombu.

Place the miso in a small bowl or measuring cup and stir in a few tablespoons of warm water until the miso dissolves, just enough to make it pourable. Stir the miso-water mixture into the pot of broth, and remove from heat. Taste, and add more dissolved miso if desire. Serve at once.