Step 1: In a medium bowl, whisk together the ketchup, rice vinegar, sugar, and pineapple juice to create the base for your sweet and sour chicken with pineapple.
Step 2: Place the chicken chunks in a bowl and coat with the beaten egg. In a separate shallow dish, combine the cornstarch and flour.
Step 3: Dredge each piece of egg-coated chicken in the flour mixture until fully covered.
Step 4: Heat oil in a large skillet or wok over medium-high heat. Fry the chicken in batches until golden brown and cooked through (about 5-6 minutes per batch). Set aside on paper towels.
Step 5: Drain most of the oil from the skillet, leaving about 1 tablespoon. Sauté the onions and bell peppers for 3 minutes until slightly softened.
Step 6: Add the pineapple chunks and the prepared sauce to the skillet. Bring to a simmer.
Step 7: Once the sauce thickens slightly, toss the crispy chicken back into the pan. Stir quickly to coat everything in the sweet and sour chicken with pineapple sauce and serve immediately.