In a medium bowl, combine the chicken pieces, 1 tbsp soy sauce, and cornstarch. Toss until the chicken is evenly coated. Set aside.

In a small bowl, whisk together the chicken broth, oyster sauce, 1 tbsp soy sauce, rice vinegar, and coarsely ground black pepper. Set aside.

Heat 1 tbsp of avocado oil in a large skillet or wok over medium-high heat. Add the chicken in a single layer (work in batches if needed) and cook for 3-4 minutes per side, until golden brown and cooked through. Remove from the skillet and set aside.

Add the remaining 1 tbsp of oil to the skillet. Add the chopped onion and celery and cook for 2-3 minutes, until crisp-tender. Add the minced garlic and ginger and cook for another 30 seconds until fragrant.

Pour the prepared sauce into the skillet and bring to a simmer, stirring constantly. Once it bubbles and thickens slightly (about 1 minute), return the cooked chicken to the skillet. Toss everything to combine and coat well with the sauce. Serve immediately with hot rice.