Pat the chicken dry and cut into thin strips. Season with 1/4 teaspoon pepper and 1/4 teaspoon salt.
Heat 1 tablespoon oil in a wok or large skillet over high heat until shimmering.
Add the chicken and cook, stirring constantly, until the chicken is cooked through and lightly golden (about 4–6 minutes).
Add the celery, green peas, and ginger; cook and stir for 30 seconds.
Add the carrot, broccoli, and garlic; cook and stir for 30 seconds.
Add the red bell pepper, pak choi (if using), baby corn, and leek; cook and stir for 30 seconds.
In a small bowl, whisk together 1 tablespoon sugar, 1 cup soy sauce (low-sodium), and 1 tablespoon cornstarch until smooth.
Pour the soy sauce mixture into the wok and cook, stirring constantly, for about 30 seconds or until the sauce thickens and coats the chicken and vegetables.
Remove from the heat and serve immediately.