Set up three shallow bowls: one with cornstarch, one with beaten eggs, and one with flour seasoned lightly with salt and pepper. Place a wire rack or paper-towel-lined plate nearby.

Pat chicken very dry with paper towels. Season with salt and pepper. Dredge in cornstarch, dip in egg, then coat in flour, shaking off excess at each step.

Heat about 1/2 inch of neutral oil in a large skillet or wok over medium-high heat until shimmering. Fry chicken in batches, 3–4 minutes per side, until golden and cooked through (165°F). Transfer to the rack to drain.

Discard most of the frying oil, leaving about 1 tablespoon in the pan. Add bell peppers and onion; sauté over medium-high heat 3–4 minutes until slightly tender but still crisp.

In a bowl, whisk together ketchup, vinegar, sugar, soy sauce, garlic powder, and the cornstarch slurry until smooth.

Pour the sauce into the skillet with the vegetables. Cook, stirring, 2–3 minutes until glossy and thickened. Add pineapple chunks if using.

Return fried chicken to the pan. Toss gently to coat every piece and simmer 2 minutes to heat through. Taste and adjust with a splash of vinegar for tang or a pinch of sugar for sweetness.

Serve immediately over steamed rice or noodles. Garnish with sesame seeds or sliced scallions if desired.