Heat 1 Tbsp oil in a large skillet or wok over medium-high heat. Add the chicken in a single layer. Work in batches if needed. Cook 2-3 minutes, stirring occasionally, until lightly browned.
Give the cornstarch slurry a quick stir and pour it into the skillet. Simmer 1-2 minutes, stirring, until the sauce thickens and coats the chicken and vegetables.