- Remove stems from mushrooms; rinse and finely chop.
- Rinse mushroom caps and thinly slice; set aside 1 cup.
- In large pot, combine mushroom stems, remaining sliced mushrooms, scallions, garlic, bay leaf, bok choy stems, tamari, ginger, pepper flakes and salt.
- Add 6 cups cold water and bring to a boil over high heat.
- Reduce heat to low, cover partially and simmer 45 minutes.
- Pour stock through strainer into clean pot or storage container; discard solids.
- In large skillet, heat sesame oil over medium-high heat.
- Add reserved mushrooms and cook, stirring often, until tender, about 3 minutes.
- Divide sauteed mushrooms and bok choy greens among soup bowls.
- Ladle some broth on top of each and serve.