Whisk ketchup, rice vinegar, brown sugar, soy sauce, and pineapple juice in a bowl. Stir in garlic and ginger.
Mix 2 tbsp cornstarch with 2 tbsp water in a small cup to make a slurry; set both sauce and slurry aside.
Season chicken with salt and pepper, toss with beaten egg, then coat evenly with the cornstarch for coating.
Heat vegetable oil in a large skillet or wok over medium-high heat until shimmering.
Pan-fry chicken in two batches until golden and cooked through, about 5–6 minutes total. Transfer to a plate.
Sauté onion and bell peppers in the same pan for 2–3 minutes until crisp-tender.
Return chicken to the pan and add pineapple. Pour in the sweet and sour sauce and bring to a simmer.
Stir in the cornstarch slurry and cook, stirring, until the sauce thickens and turns glossy, 1–2 minutes.
Serve hot over steamed rice or noodles.