Prep the chicken. Pat the chicken pieces dry. In a bowl, whisk the egg, soy sauce, rice vinegar, garlic powder, and pepper. Add the chicken and toss to coat. Let sit 10–15 minutes while you prep everything else.

Mix the breading. In a shallow bowl, combine cornstarch, flour, and salt. Dredge the marinated chicken in the mixture, pressing lightly so the coating adheres. Shake off excess and place on a plate.

Make the sauce base. In a small bowl or measuring cup, mix ketchup, rice vinegar, sugar, pineapple juice, and soy sauce. Stir until the sugar mostly dissolves. Set aside. In another small cup, stir cornstarch with water to make a slurry.

Cook the vegetables. Heat 1 tablespoon oil in a large skillet or wok over medium-high heat. Add bell peppers and onion. Stir-fry 2–3 minutes until slightly tender but still crisp. Add pineapple and toss for 30 seconds. Transfer to a plate.

Fry the chicken. Add enough oil to the skillet to shallow fry (about 1/4 inch). Heat over medium-high until shimmering. Fry the chicken in batches, 3–4 minutes per side, until golden and cooked through. Don’t overcrowd the pan. Transfer to a wire rack or paper towels.

Make the sauce. Pour off excess oil, leaving 1–2 teaspoons in the pan. Return the pan to medium heat. Pour in the sauce base and bring to a simmer. Stir the cornstarch slurry, then add it gradually, stirring constantly, until the sauce thickens and turns glossy, about 30–60 seconds.

Combine. Add the cooked chicken, peppers, onion, and pineapple to the pan. Toss until everything is evenly coated. If the sauce gets too thick, splash in 1–2 tablespoons water to loosen.

Taste and adjust. Add a pinch of salt if needed. For more tang, add a splash of vinegar; for more sweetness, add 1–2 teaspoons sugar. Remove from heat.

Serve. Spoon over hot steamed rice or alongside noodles. Garnish with sesame seeds and scallions if you like.