In a bowl, prepare the Marinade. Lightly beat the egg white, then add the salt and ground white pepper. Marinate the chicken with this mixture and set aside. This will ensure a smooth and silky texture when the chicken is cooked. Discard any excess marinade.
Mix all the ingredients for the White Sauce together and stir until well combined. Set aside.
In a wok or skillet over high heat, add two tablespoons of oil and heat until hot. Add the chicken and stir-fry until it is just over halfway cooked, about 45 seconds, or until the surface turns white. Remove from the pan, drain on paper towels, and set aside. Clean and rinse the wok or skillet.
Heat the wok or skillet on high heat again. Add the remaining 1 tablespoon of oil and stir-fry the garlic until it is light brown. Toss in the mixed vegetables and continue to stir-fry for 1 minute. Then add the mushrooms and stir to combine well.
Add the chicken and the White Sauce to the wok or skillet. Stir to combine well with all the ingredients, cooking for about 1-2 minutes, or until the chicken is cooked through. Dish out and serve immediately with steamed rice.