Add 1 tablespoon soy sauce, 1 tablespoon dry sherry, 1 tablespoon cornstarch, and plenty of freshly cracked black pepper to medium mixing bowl. Whisk ingredients together until fully incorporated.
Add 1 ½ pounds boneless, skinless chicken breasts, cut into bite-sized pieces, to mixing bowl. Toss or stir chicken until fully coated in marinade.
Set bowl aside and let chicken marinate 10 to 15 minutes.
While chicken marinates, add ½ cup low-sodium chicken stock, 2 tablespoons dry sherry, 1 tablespoon oyster sauce, 3 tablespoons soy sauce, 1 tablespoon granulated sugar, 1 tablespoon cornstarch, and 2 teaspoons coarsely ground black pepper to other medium mixing bowls. Whisk ingredients together until fully combined. Set sauce aside.
After chicken has marinated 10 to 15 minutes, place large skillet on stovetop over high heat. Add 1 tablespoon avocado oil to skillet and continue heating pan until oil is hot and shimmery.
When oil is hot, add marinated chicken to skillet, arranging chicken in one even layer with little-to-no overlap if possible.
Sear chicken 30 seconds or until undersides of chicken pieces are lightly browned.
Once lightly browned underneath, quickly flip chicken over and sear chicken on other side 20 seconds.
After 20 seconds, stir chicken and continue cooking until chicken is just lightly browned on all sides.
Transfer browned chicken to large plate and set aside.
Reduce heat under skillet to medium-high. Add remaining 1 tablespoon avocado oil to skillet. Heat pan until oil is hot and shimmery.
When oil is hot, add ½ cup chopped white onion, ½ cup chopped green bell pepper, ½ cup chopped red bell pepper, and ¼ cup chopped celery to pan. Stir to incorporate.
Sauté vegetables 60 seconds, stirring occasionally.
After 60 seconds, add prepared sauce mixture to skillet and stir to incorporate. Simmer sauce until thickened, approximately 15 to 30 seconds.
Return chicken to skillet and stir to incorporate, coating chicken and vegetables completely in sauce.
Portion chicken and vegetables onto serving plates. Spoon sauce from pan over chicken then serve hot with desired sides.