Place the millet in a large bowl and rinse with water 2-3 times (until the water runs almost clear). Drain the water and set it aside.
Remove the shiitake mushrooms from the water which it was rehydrated in (save the mushroom soaking water for Step 4) and thinly slice the mushrooms. Set aside.
Thinly slice the chicken thighs and set aside.
Combine the 4 cups of water with the 2 cups of reserved mushroom water (strain the mushroom water to remove any impurities) and pour into a pot. Bring the liquid to a boil. Once boiled, add the millet.
Bring it back up to a boil and then add the mushrooms, chicken thighs, and ginger. Turn the heat down to low and cover the pot. Cook for 30 minutes.
Stir the pot every 5-10 minutes to make sure the millet stick to the bottom of the pot, and to help make everything cook evenly. Skim any foam that rises to the top.
Once the porridge / congee is done cooking, season with salt and white pepper. Then spoon into individual bowls and top with green onions. Eat and enjoy ^_^