Boil dried shiitake mushrooms in water in covered pot for 4 minutes.
Remove from heat, strain mushrooms from liquid and set mushrooms aside.
Mix together dry ingredients and add slowly to broth while mixing with an immersion blender. Mix until all dry ingredients are completely dispersed / dissolved.
Add mushrooms back to broth and also add cubed tofu. Top with thinly sliced green onions before serving, if desired.