Prepare all ingredients by chopping chicken, vegetables, and pineapple.
Season the chicken pieces with salt and pepper.
Dredge the chicken in cornstarch until fully coated.
Heat vegetable oil in a pan over medium heat.
Add the chicken and cook until golden brown, about 5-7 minutes.
Remove chicken from the pan and set aside.
Add onion and bell peppers to the same pan and sauté for 3-4 minutes.
Return the chicken to the pan and add the pineapple chunks.
Pour sweet and sour sauce over the chicken and vegetables.
Stir well and cook for an additional 5 minutes until heated through.