Combine chicken pieces, vegetable oil, and salt in a big bowl. Mix well and let marinate for 10 to 15 minutes.
Mix all the ingredients for the sauce in a small bowl.
Add the beaten egg into the bowl with the chicken. Stir to mix well. Add cornstarch. Stir to coat the chicken, until it forms an uneven coating with a little dry cornstarch left unattached.
Heat oil in a large nonstick skillet over medium high heat until hot. Add chicken all at once and spread out into a single layer in the skillet. Separate chicken pieces with a pair of tongs or chopsticks.
Cook without touching the chicken for 2 to 3 minutes, or until the bottom turns golden. Flip to brown the other side, 2 to 3 minutes. Transfer chicken to a big plate and remove the pan from the stove. Let cool for 2 to 3 minutes.
Wipe the extra oil from the pan using a few layers of paper towel held in a pair of tongs, leaving about 1 tablespoon of oil in the pan. Add the garlic, ginger, and the white part of the green onion. Stir fry a few seconds to release the aroma.
Stir the sauce again to thoroughly dissolve the cornstarch.
Add the chicken back into the pan and pour in the sauce. Stir to mix well.
Transfer to a plate, garnish with the green part of the green onion, and serve hot over steamed rice. You can add some steamed vegetables on the side to add some color to the dish.