Heat the vegetable broth in a large pot over high heat. When the broth comes to a simmer, lower the heat to medium and add the carrots, ginger and scallions.
Add the tofu, if using, and cook for 5 more minutes. Stir in the miso paste, peas, mushrooms, and ponzu sauce and cook for another minute, until the peas are hot.
Meanwhile cook the noodles according to package directions, drain, and rinse with cold water. Add them to the soup and heat until just barely simmering. Divide the soup into 4 bowls and serve hot.
Japanese wheat noodles are very easy to overcook, so for this soup they get cooked on their own, then I just decided to give them a rinse in cold water to stop the cooking. Then I added them to the soup to reheat just before serving. That’s the way I’ve found it easier to not let them get mushy.