Start by making the stir fry sauce. In a small bowl combine the soy sauce, sweet red chili sauce, brown sugar, rice vinegar, toasted sesame oil, cornstarch and water. Stir together until well combined and set the sauce to the side.
Next, chop the onion and bell pepper. Chop the broccoli into small florets. And mince the garlic cloves. Set the veggies to the side.
Place the chicken breasts on a cutting board, cover them with plastic wrap or parchment paper, and pound them until they are an even thickness. Then cut the chicken breasts into roughly 1-inch pieces. Season the chicken with the salt and black pepper.
Now in a large 12-inch skillet, heat 2 tablespoons of oil over medium-high heat and add the chicken pieces. Cook the chicken until cooked through, about 3 minutes per side. Once the chicken is done, remove it from the skillet to a separate plate and set it aside.
In the same skillet, add the remaining 1 tablespoon of oil. Add the broccoli to the skillet and sauté for 1 minute. Now add the onion and bell pepper. Sauté for 2-3 more minutes. Now add the minced garlic to the skillet and saute for another 30 seconds. At this point the vegetables should be tender, but still slightly crisp.
Add the cooked chicken back to the skillet and stir with the vegetables. Give the stir fry sauce another stir then pour the sauce over the chicken and vegetables. Mix everything together making sure all of the chicken and veggies are coated in the sauce. Lower the heat to medium-low and simmer for 2 minutes.
Once the stir fry is done, turn the heat off, top with sliced green onions and serve with rice or noodles. Enjoy!