Cook udon as package directs, rinse and drain. Set aside. Remove stems from the shiitake and discard. Reserve soaking water. Slice caps. Place 4 cups water, including the shiitake soaking water and sliced shiitake in a medium sauce pan. Cover and bring to a boil. Simmer for 5 minutes. Add the wakame flakes, carrots and tofu. Simmer 1 minute. Puree the miso with a little soup broth. Reduce flame to low, add the miso and simmer 2 minutes. Place the cooked udon and the watercress in the pot to heat up for 1 minute. Place the udon in serving bowls, ladle the hot soup over and garnish with scallions.
Per serving - 296 calories, 6 g fat (17% calories from fat), 18 g protein, 44 g carbohydrate, 6 g fiber, 0 mg cholesterol, 526 mg sodium