Prepare the Chicken:

1. Coat the Chicken: Place the cornstarch in a medium bowl, and season well with salt and pepper. Toss the chicken pieces in the cornstarch mixture until they are evenly coated. Ensure that every piece of chicken is coated for even cooking and crispiness. The cornstarch not only helps the chicken to crisp up nicely but also acts as a thickening agent for the sauce.

Cook the Chicken:

2. Heat the Oil: In a large, high-sided skillet or wok, heat the vegetable oil over medium-high heat. The oil is hot enough when a small piece of chicken sizzles immediately upon contact. Using a wok allows for even heat distribution and makes stir-frying easier.
3. Fry the Chicken: Gently add the chicken pieces to the hot oil, ensuring not to overcrowd the pan. Fry the chicken on all sides until it is golden brown. Remove the chicken from the skillet and place it on a plate lined with paper towels to drain any excess oil. The chicken does not need to be fully cooked at this stage; it will finish cooking in the sauce later. Overcrowding the pan will lower the oil temperature and prevent the chicken from browning properly. It will also cause the chicken to steam instead of fry, resulting in a less crispy texture. It’s best to fry the chicken in batches if necessary.

Cook the Vegetables:

4. Sauté the Onion: After frying the chicken, add the sliced onion to the skillet. Scrape up the browned bits from the bottom of the skillet using a spatula or wooden spoon. These browned bits, called “fond,” add a lot of flavor to the sauce. Sauté the onion until it begins to soften and become translucent.
5. Add the Bell Peppers: Add the chopped bell peppers to the skillet and cook until they start to soften, about 5-7 minutes. You want the vegetables to be tender-crisp, not mushy. Cooking them over medium heat will prevent them from burning.

Prepare the Sauce:

6. Combine Sauce Ingredients: In a bowl or measuring jug, whisk together the ketchup, cider vinegar, brown sugar, soy sauce, grated ginger, and grated garlic. This is the base of your sweet and sour sauce! Whisking the ingredients together ensures that they are evenly distributed and prevents any lumps from forming.
7. Simmer the Sauce: Pour the sauce into the skillet and bring it to a gentle simmer. Simmering the sauce allows the flavors to meld together and deepen.
8. Add Pineapple and Juice: Add the drained pineapple pieces to the skillet, reserving the juice. Adjust the sauce consistency by adding pineapple juice to taste. You can add more or less juice depending on how thick you like your sauce. If you prefer a thicker sauce, simmer it for a longer period.

Combine and Cook:

9. Return Chicken to Skillet: Add the partially cooked chicken back into the skillet with the sauce and vegetables.
10. Simmer and Finish Cooking: Simmer the mixture for another 5-6 minutes, or until the chicken is cooked through (internal temperature of 165°F/74°C) and the sauce has thickened. The sauce will thicken as it simmers. Stir the chicken occasionally to ensure that it is evenly coated in the sauce.