In a large skillet or wok, heat 2 tbsp of oil over medium-high heat. In batches so that you do not overcrowd the pan, brown the chicken on both sides until a golden brown crust forms and the chicken is just cooked through. Transfer cooked chicken to a plate and continue until all of the chicken is browned, adding more oil to the skillet as needed throughout the cooking process.
Once all the chicken is browned and set aside, add the sliced white and light green parts of the green onions (save the dark green pieces until the end), the ginger, and the garlic and saute, stirring, for only 20-30 seconds being careful not to burn. Immediately pour in the prepared sauce mixture and let come to a rapid simmer. Cook, stirring, until the sauce has thickened, 2 to 3 minutes.
Add the chicken back into the skillet along with the red chili flakes (if using) and the dark green pieces of the green onion. Toss so that the chicken is well coated in the sauce and cook just long enough to heat the chicken back up so it's nice and hot for serving.
Remove from heat and serve immediately.