Season the chicken with salt and pepper, then toss in 1 tablespoon cornstarch.
Heat oil in a large skillet or wok over medium heat. Add chicken and cook until golden and cooked through. Remove and set aside.
In the same pan, sauté bell peppers and onion until slightly softened.
Add pineapple chunks and stir for another minute.
In a small bowl, whisk together ketchup, soy sauce, rice vinegar, brown sugar, water, and cornstarch.
Pour sauce into the pan, bring to a simmer, and cook until it thickens.
Return the chicken to the pan, toss everything to coat, and cook for another minute until heated through.
Serve hot over rice (or just eat it straight from the pan—I won’t judge).