Step 1: Prepare the chicken. In a medium bowl, combine chicken pieces with salt and pepper. In another shallow bowl, whisk the egg. In a third shallow bowl, combine cornstarch and flour. Dip each chicken piece first into the egg, letting excess drip off, then dredge thoroughly in the cornstarch-flour mixture, ensuring it’s fully coated. Set aside on a wire rack.
Step 2: Prepare the sweet and sour sauce. In a medium bowl, whisk together the pineapple juice, rice vinegar, granulated sugar, ketchup, soy sauce, cornstarch, and water until well combined and no lumps remain. Set aside.
Step 3: Fry the chicken. Heat about 1 inch of oil in a large skillet or wok over medium-high heat to 350-375°F (175-190°C). Carefully add chicken pieces in batches, making sure not to overcrowd the pan. Fry for 4-6 minutes per batch, turning occasionally, until golden brown and cooked through. Remove chicken with a slotted spoon and place on a wire rack set over a paper towel-lined baking sheet to drain excess oil. Repeat with remaining chicken.
Step 4: Cook the vegetables. Discard all but 1 tablespoon of oil from the skillet. Add the red and green bell peppers to the hot skillet and stir-fry for 2-3 minutes until slightly tender-crisp. Add the pineapple chunks and cook for another minute.
Step 5: Combine with sauce. Give the prepared sweet and sour sauce mixture another quick whisk. Pour the sauce into the skillet with the vegetables and pineapple. Bring to a simmer, stirring constantly, until the sauce thickens and becomes glossy, about 2-3 minutes.
Step 6: Add chicken back. Return the fried chicken to the skillet with the sauce and vegetables. Toss gently until all the chicken pieces are thoroughly coated in the sweet and sour sauce.
Step 7: Serve immediately. Garnish with chopped green onions and sesame seeds, if desired. Serve this delicious sweet and sour chicken recipe hot with steamed white rice or noodles.