Step 1: Prepare the Chicken. In a medium bowl, toss the chicken pieces with salt and pepper. In another bowl, whisk together the cornstarch and flour. In a third bowl, whisk the egg and milk.
Step 2: Dredge the Chicken. Dip each chicken piece first into the egg wash, then dredge it thoroughly in the flour/cornstarch mixture, ensuring it's fully coated. Set aside on a clean plate. This double coating is key to a crispy sweet and sour chicken recipe.
Step 3: Fry the Chicken. Heat 2-3 cups of vegetable oil in a large deep skillet or Dutch oven over medium-high heat to 350°F (175°C). Fry chicken in batches, being careful not to overcrowd the pan. Cook for 4-6 minutes per batch, or until golden brown and cooked through. Remove with a slotted spoon and place on a wire rack set over a paper towel-lined baking sheet to drain excess oil. Keep warm.
Step 4: Prepare the Sweet and Sour Sauce. In a medium saucepan, combine sugar, rice vinegar, ketchup, pineapple juice, and soy sauce. In a small bowl, whisk together 1 tablespoon cornstarch with 1/2 cup water until smooth to create a slurry. Add the slurry to the saucepan. Bring the sauce to a simmer over medium heat, stirring constantly, until it thickens, about 2-3 minutes. Set aside.
Step 5: Stir-Fry Vegetables. Heat 1 tablespoon of vegetable oil in a large wok or skillet over medium-high heat. Add red bell pepper, green bell pepper, and onion. Stir-fry for 3-4 minutes, until vegetables are tender-crisp. Add the pineapple chunks and cook for another minute.
Step 6: Combine and Serve. Add the fried chicken pieces to the wok with the vegetables. Pour the sweet and sour sauce over the chicken and vegetables. Toss gently to coat everything evenly. Cook for 1-2 minutes, just long enough for the chicken to heat through and be thoroughly coated in the glorious sweet and sour sauce. Garnish with sesame seeds and green onions if desired.
Step 7: Serve immediately with hot white rice. Enjoy your homemade sweet and sour chicken recipe!