Pat the chicken pieces dry with paper towels. In a bowl toss the chicken with 1 tsp salt, 1 tsp black pepper, and 1 tsp garlic powder until evenly coated.

Coat with cornstarch

Place 1 cup cornstarch in a shallow dish. Dredge each seasoned chicken piece in the cornstarch, pressing lightly so a thin even coating sticks. Shake off excess and place on a tray.

Fry the chicken

Heat 1 cup vegetable oil in a large skillet or wok over medium-high heat until shimmering (about 350°F / 175°C if using a thermometer). Fry the coated chicken in batches so pieces do not crowd the pan, 3–4 minutes per batch, turning once, until golden and cooked through (internal temperature 165°F / 74°C). Transfer to a paper-towel-lined plate to drain. Keep warm.

For the sauce

Combine sauce ingredients

In a bowl or measuring cup combine 1 cup pineapple juice, 3 tbsp rice vinegar, 3 tbsp ketchup, 2 tbsp light soy sauce, 3 tbsp brown sugar, and 1 tbsp hoisin sauce (if using). Stir until sugar is mostly dissolved.

Make cornstarch slurry

In a small cup whisk together 2 tsp cornstarch with 2 tbsp cold water until fully smooth and no lumps remain.

Cook sauce

Wipe out the frying pan or use a clean wok, heat over medium. Pour the combined sauce mixture into the pan and bring to a gentle simmer, stirring. After it simmers for 1–2 minutes add the cornstarch slurry while whisking; continue to simmer until the sauce thickens to a glossy coating that clings to a spoon, about 1–2 minutes. Taste and adjust: add up to 1/2 tsp salt or more vinegar/sugar to balance sweet-tart to preference.

Vegetables & finishing

Stir-fry vegetables

Heat 1 tbsp vegetable oil in a clean wok or large skillet over medium-high heat. Add the onion and stir-fry 1 minute, then add the red and green bell peppers and continue to stir-fry 2–3 minutes until peppers are bright and just tender-crisp.

Add pineapple and chicken

Add 1 cup pineapple chunks to the vegetables and stir 30 seconds to warm. Return the fried chicken pieces to the wok and pour the thickened sauce over everything. Toss or gently stir to coat all pieces, cooking 1–2 minutes more so flavors meld and chicken is heated through.

Garnish and serve

Transfer to a serving plate or bowl. Sprinkle with sliced green onions and 1 tsp sesame seeds (if using). Serve immediately with steamed rice or noodles.