Heat the vegetable broth in a medium saucepan over medium heat. Add the mushrooms, carrots, scallions and garlic. Bring the vegetables to just under a boil, and then reduce the heat to medium-low and simmer, covered, for 3 minutes. Next, add the bok choy, tamari and cinnamon and simmer, covered, for 2 minutes.

Drain the rice noodles, add them to the soup and cook for about 5 minutes. Remove the saucepan from the heat and stir in the red pepper flakes. Ladle the soup into individual bowls and garnish each bowl with cilantro. Serve at once.