Mix the chicken pieces in a bowl with 1 tsp cornstarch, 1 Tbsp oil, 1/2 tsp salt and 1/2 tsp pepper. Let sit. Pour the boiling stock over the mushrooms in another bowl and stir in 2 tsp cornstarch, 1/2 tsp salt and 1/2 tsp pepper. Quick fry the chicken, over high heat for 30 seconds then add the mushroom mixture and the ginger. Stir constantly until just done, no more than 5 minutes, over medium heat. Serve immediately.