Toss sliced chicken with cornstarch and salt until evenly coated.
Heat oil in a large skillet or wok over medium-high heat. Add chicken in a single layer and cook until lightly golden and cooked through. Remove and set aside.
In the same pan, add garlic and ginger; cook until fragrant, about 30 seconds.
Stir in soy sauce, brown sugar, and water. Bring to a simmer until slightly thickened and glossy.
Return chicken to the pan and toss to coat evenly in the sauce.
Add green onions and optional red pepper flakes. Stir briefly and remove from heat.
Serve immediately over rice or noodles.