Gently rinse the dried shiitake mushrooms with tap water to remove any dust. Add the mushrooms into a medium-sized bowl and add 1/2 cup boiling water to cover. Make sure the mushrooms are submerged. Let rehydrate until the mushrooms turn soft, 15 to 20 minutes. Cut and prepare the other ingredients in the meantime. Once the mushrooms are done soaking, squeeze out the excess liquid and slice into strips. Reserve the soaking liquid.