In a medium bowl, whisk together the soy sauce, chicken stock, brown sugar, black pepper and rice wine vinegar. Set the sauce aside.
Add 2 tablespoons oil to a large skillet or wok set over medium-high heat. Once the oil is hot, add the chicken and cook until cooked through, about 5 minutes. Using a slotted spoon, transfer the chicken to a plate.
Add the remaining 1 tablespoon oil to the skillet along with the onions, peppers and garlic. Cook, stirring, until the onion is translucent, about 2 minutes.
Add the prepared sauce to the skillet and cook for 1 minute then bring the mixture to a boil and stir in the cornstarch mixture. Boil the mixture until the sauce is thick and syrupy, about 2 minutes.
Return the chicken and all juices to the skillet and toss until the chicken is coated in the sauce.
Garnish with cilantro and serve with rice or veggies.