In a bowl, toss chicken with 1 tbsp soy sauce, 1 tbsp oyster sauce, and cornstarch. Let marinate for 10 to 15 minutes.
In a wide-mouth jar, add remaining 2 tbsp soy sauce, remaining 1 tbsp oyster sauce, rice vinegar, water, honey, and black pepper. Shake to combine and set aside.
Heat 1 tbsp sesame oil in a large pan over medium-high heat. Add marinated chicken and cook for 3 to 4 minutes, until browned and mostly cooked through. Remove from the pan.
Add remaining 1 tbsp oil to the same pan. Stir in onion, bell pepper, and celery. Cook for 2 to 3 minutes until starting to soften. Add garlic and ginger, and cook for another 30 seconds.
Return the chicken to the pan. Pour in the sauce and toss everything together. Cook for 2 to 3 more minutes, until sauce thickens and coats the chicken and veggies. Taste and adjust pepper or salt if needed.
Serve over rice and garnish with sliced green onions or sesame seeds, if using. Best served hot!
