Prepare the chicken by cutting it into bite-sized pieces and seasoning with salt and pepper.
In a large skillet or wok, heat vegetable oil over medium-high heat. Add the chicken pieces and cook until browned.
Remove the chicken from the skillet and set aside. In the same skillet, add onions and bell peppers.
Add pineapple chunks and cook for another 2-3 minutes until warmed through.
In a bowl, combine sugar, vinegar, soy sauce, and cornstarch, mixing until smooth.
Pour the sauce mixture into the skillet, stirring to combine all ingredients.
Return the chicken to the skillet, mixing to coat everything evenly with the sauce.
Cook for an additional 2-3 minutes, allowing the sauce to thicken and glaze the chicken and veggies.
Serve the sweet and sour chicken over rice or noodles, garnished with sesame seeds or green onions if desired.
