Add the vegetable stock, ginger, cinnamon, and soy sauce to a large pot, and bring to a boil over high heat. Reduce heat to low, cover, and simmer for at least 10 minutes.
Meanwhile, slice the mushrooms and white part of the green onions, and chop the Bok choy.
Remove the spices from the broth with a slotted spoon.
Add the mushrooms, the whites of the onions, and Bok choy to the broth. Simmer, covered, for 10-15 minutes, until mushrooms are cooked through.
Turn off the heat.
Measure the miso paste into a small bowl, and ladle some broth into it. Whisk until miso paste is dissolved into broth, and pour into the pot. Stir to combine.
Garnish with sliced onion greens and sesame seeds if desired.