To make the sauce, mix ketchup, pineapple juice, sugar, rice vinegar and soy in a small pot and mix to combine. Cook over medium heat for until it comes to a boil. Mix cornstarch with water then add to the pot and cook just until the sauce has thickened then remove from heat and set aside.
Season chicken with salt and black pepper then coat with cornstarch. In about 2 inches of oil fry the chicken until crispy, about 7-10 minutes then place on a plate lined with paper towel to drain.
Remove all but 1 tsp of oil then add onion and pepper and sauté for a minute before adding chicken and sweet and sour sauce. Stir to combine and cook for an additional 3-5 minutes. Remove from heat and garnish with chopped scallion.