Begin by cutting the chicken breast into bite-sized pieces.
In a bowl, mix cornstarch and salt, then coat the chicken pieces.
Heat oil in a large skillet over medium-high heat.
Add the chicken to the skillet and cook until golden brown.
Remove the chicken and set aside.
In the same skillet, add bell pepper and onion, sautéing for a few minutes.
In a separate bowl, combine ketchup, vinegar, and sugar.
Pour the sauce over the vegetables and bring to a simmer.
Return the chicken to the skillet and stir to coat with the sauce.
Cook for an additional 5 minutes until heated through.