Heat a skillet or wok over medium-high heat. Once the skillet is hot, transfer the marinated chicken to it and let it simmer, stirring occasionally, until golden brown (about 3-5 minutes). Transfer the chicken to a bowl and set aside.
If necessary, add a bit of oil to the pan. Add the vegetables, and let them sauté until they've softened—about 5 minutes.
Bring the chicken back to the pan and cook alongside the vegetables for another minute or until the chicken registers an internal temperature of 165ºF. Stir in the sweet and sour sauce and toss all of the ingredients to evenly coat them. Bring the skillet to a low simmer and let the sauce thicken a bit.
Turn off the heat, stir in the scallions, and top with sesame seeds.
Serve with rice, noodles, or as is!