Make the marinade: In a large shallow dish, add the soy sauce, rice wine (or sherry), sesame oil, garlic, black pepper, and salt and whisk to combine. Add 6 tablespoons of cornstarch and whisk to combine, breaking up any lumps of cornstarch. Add the diced chicken to the cornstarch mixture and toss to combine. Spread into a single layer and let sit for 15 to 20 minutes.

Make the sauce: Meanwhile, in a separate bowl, whisk to combine all of the sauce ingredients. Set aside.

When chicken is done marinating, sprinkle the pieces with remaining 6 tablespoons of cornstarch, tossing to coat. (Some of the pieces may stick together, which is fine.) Heat a wok or a large nonstick skillet over medium-high heat. Add 2 tablespoons of neutral oil. Working in batches, add the chicken pieces to the hot oil, separating any pieces that are stuck together. Continue to cook until chicken is golden brown with a thin crispy crust and is completely cooked though, 4 to 5 minutes. Place chicken pieces in a single layer on a paper towel-lined sheet pan.

Add 1 tablespoon of oil to the wok/skillet and heat over high until you see a wisp of smoke. Add the bell peppers and celery and season with a pinch of salt. Cook, tossing frequently over high heat for about 2 minutes. (The bell peppers should begin to blister.)

Add sliced garlic and scallion whites and cook for another 2 minutes, continuing to toss and stir to make sure the heat is evenly distributed. Add the stir fry sauce and bring to a rapid boil so that sauce begins to reduce. Continue cooking until the sauce has reduced by half, about 3 minutes.

Working in batches, return chicken to wok/skillet, tossing to coat with sauce after each addition. Top with scallion greens and serve with steamed white rice.