Combine chicken pieces, vegetable oil, and salt in a big bowl. Mix well and let marinate for 10 to 15 minutes.
Combine all the sauce ingredients in a small bowl and set aside.
Add the beaten egg into the bowl with the chicken. Stir to mix well. Add cornstarch. Stir to coat 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 and green onion. Stir fry a few seconds to release the aroma.
Stir the sauce again to thoroughly dissolve the cornstarch. Add to the pan and cook until thickened.
Add the chicken back into the pan and stir to mix well.
Transfer to a plate and serve hot over steamed rice.