Cut the chicken breasts into small bite-sized pieces and place them in a bowl. Add the cornstarch and 1 tablespoon soy sauce. Mix well until the chicken is coated.
Heat the oil in a large pan over medium-high heat.
Add the chicken pieces to the pan and cook for 5–7 minutes until golden and fully cooked. Stir occasionally.
Add the chopped onion, bell pepper, and minced garlic. Cook for 2–3 minutes until slightly soft.
Add the pineapple chunks and mix everything together.
In a small bowl, combine the ketchup, vinegar, sugar, 1 tablespoon soy sauce for the sauce, and water. Stir well.
Pour the sauce into the pan and mix everything together.
Cook for 3–4 minutes while stirring until the sauce becomes slightly thick and coats the chicken.
Turn off the heat and serve hot with rice or noodles.