1. In a mixing bowl, combine the chicken slices with cornstarch. Toss to coat evenly.
2. Heat the vegetable oil in a wok over medium-high heat. Add the chicken and stir-fry until golden brown and cooked through. Remove chicken from the wok and set aside.
3. In the same wok, add garlic and ginger. Stir-fry for about 30 seconds until fragrant.
4. Add soy sauce, hoisin sauce, and brown sugar to the wok. Stir well to combine.
5. Return the chicken to the wok and toss to coat with the sauce. Cook for another 2-3 minutes until the sauce thickens.
6. Garnish with chopped green onions and serve hot.