Chicken Teriyaki Stir-Fry

In a small bowl, whisk together the soy sauce, brown sugar, rice vinegar, ginger, and garlic to make the teriyaki sauce; set aside.
Heat 1 tbsp of vegetable oil in a large wok or skillet over high heat until it shimmers and just starts to smoke lightly.
Add the chicken pieces in a single layer and let them sear undisturbed for 2 minutes until golden on the bottom before stirring.
Continue stir-frying for 4 to 5 minutes total, until the chicken is opaque throughout and reaches 165F, then transfer to a plate.
Add the remaining 1 tbsp oil to the same pan, then add the broccoli, bell pepper, and carrots.
Stir-fry the vegetables over high heat for 4 to 5 minutes, tossing frequently, until they are bright in color and crisp-tender, not soft or mushy.
Return the cooked chicken to the pan and pour in the teriyaki sauce, stirring to coat everything evenly.
Bring the sauce to a simmer, then stir in the cornstarch slurry and cook for 1 to 2 minutes until the sauce thickens into a glossy glaze that coats the back of a spoon.
Remove from heat once everything is well coated and glossy; taste and add a splash more soy sauce if needed.
Sprinkle with sesame seeds and green onions, then serve immediately over steamed rice.

Searing the chicken undisturbed for the first 2 minutes is what gives it color instead of steaming gray.

Do not overcook the vegetables; pulling them while still bright and crisp keeps the stir-fry from turning mushy.