Cut any fat off of the breasts, and then carefully cut the breasts into roughly 1" cubes. Transfer the chicken to a mixing bowl.
In a small prep bowl, combine the cornstarch, salt, & pepper until evenly combined. Sprinkle the cornstarch mix evenly out over the chicken, and toss until evenly coated.
Add the oil to a large skillet set over medium heat. Once the oil's hot (shimmery), add 1/3 of the prepared chicken to the skillet. Spread it out and let it cook, stirring as needed. It will cook quickly. Using a slotted spoon, transfer the cooked chicken to a waiting plate, and repeat until all of the chicken's cooked.
Return all of the chicken and any juices to the skillet. Stir the vegetables in, and cook until they're just tender- roughly 5 minutes.
Pour the chicken broth & sauce mixture into the skillet, gently tossing the chicken & veggies to coat.
Turn the heat up to medium high, and bring the sauce to a boil. Immediately reduce the heat, and let the mixture simmer for 8-10 minutes. Turn off the heat, stir well, and let the mixture rest for 1-2 minutes before serving.
Spoon the mixture over steamed white rice, or side by side with the rice. Enjoy!