Prepare the Chicken: Start by cutting the boneless, skinless chicken breasts into bite-sized pieces. Place the chicken in a bowl.
Marinate the Chicken: In a separate bowl, whisk together the soy sauce, rice vinegar, tomato ketchup, brown sugar, grated ginger, minced garlic, salt, and black pepper. Pour the marinade over the chicken pieces, ensuring they are well coated. Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to infuse.
Prepare the Batter: In a shallow dish, combine the all-purpose flour, cornstarch, and baking powder. Mix well to ensure the ingredients are evenly incorporated.
Coat the Chicken: Remove the marinated chicken from the refrigerator. Dip each piece of chicken into the batter, ensuring it is fully coated. Shake off any excess batter and set aside on a plate.
Heat the Oil: Pour vegetable oil into a deep pan or wok, filling it about one-third of the way. Heat the oil over medium-high heat until it reaches approximately 350°F (175°C). To test if the oil is hot enough, drop a small amount of batter into the oil. If it sizzles and rises to the surface, the oil is ready for frying.
Fry the Chicken Balls: Carefully place the coated chicken balls into the hot oil, a few at a time, making sure not to overcrowd the pan. Fry them for about 4-5 minutes or until they turn golden brown and crispy. Use a slotted spoon or tongs to remove the chicken balls from the oil and transfer them to a paper towel-lined plate to drain excess oil.
Make the Sweet and Sour Sauce: In a separate saucepan, combine the remaining marinade with 1/2 cup of water. Bring the mixture to a boil over medium heat, then reduce the heat and simmer for 5 minutes until the sauce thickens slightly. If desired, you can add pineapple chunks to the sauce for extra flavor.
Stir-Fry the Vegetables: In a clean wok or skillet, heat a tablespoon of vegetable oil over medium-high heat. Add the bell pepper and onion chunks and stir-fry for a few minutes until they are crisp-tender. You can also add pineapple chunks at this stage if you didn't include them in the sauce.
Combine and Serve: Add the fried chicken balls to the wok or skillet with the stir-fried vegetables. Pour the sweet and sour sauce over the chicken and vegetables, tossing gently to coat everything evenly. Cook for an additional 1-2 minutes until the chicken and vegetables are fully coated and heated through.
Garnish and Enjoy: Transfer the sweet and sour chicken balls to a serving dish. Garnish with sliced spring onions for a fresh burst of flavor and vibrant color. Serve the dish hot over steamed rice or noodles, and enjoy the delightful combination of flavors and textures!