Soak the mushrooms: Soak dried shiitake mushrooms in cold water until softened (3–5 hours). For a faster method, use hot water, though it will be slightly less fragrant. Once softened, slice and squeeze out any excess water.
Marinate the chicken: In a mixing bowl, combine diced chicken with salt, cooking wine, ginger, white pepper, soy sauce, oil, and flour. Add the sliced mushrooms and mix well. Marinate for at least 1 hour, or overnight for deeper flavour.
Steam: Spread the marinated chicken and mushrooms evenly in a shallow heatproof dish (don’t overcrowd). Steam over boiling water for 12–15 minutes, until the chicken is fully cooked through.
Serve: Carefully uncover, sprinkle chopped spring onion on top, and enjoy while hot.