Place the dried shiitake mushrooms in a small heat proof bowl. Pour boiling water until the mushrooms are submerged and allow to soak for about 20 minutes.
Squeeze out the excess water from the mushrooms. Reserve the liquid. Thinly slice the mushrooms
In a large saucepan, place the onions and bok choy (cut side down) in a single layer. If they do not all fit, divide into two batches. Sear at medium high heat until the bok choy and onions carmelize (about 3 minutes).
Add reserved mushroom water, ginger, garlic, and sliced mushrooms. Cover and allow to steam until all liquid has been absorbed or until desired done-ness.