Prep the chicken: Pat the chicken dry and slice into thin strips. Season lightly with salt and pepper. This helps it brown quickly and stay tender.
Mix the sauce: In a bowl, whisk soy sauce, pineapple juice, vinegar, brown sugar, ketchup, sesame oil, and red pepper flakes.
Stir in the cornstarch slurry. Set aside.
Heat the pan: Use a large wok or skillet. Heat 1 tablespoon of oil over medium-high until shimmering.
Cook the chicken in batches: Add half the chicken in a single layer.
Sear 2–3 minutes per side until just cooked through. Remove to a plate. Repeat with the remaining chicken, adding oil if needed.
Don’t overcrowd the pan.
Stir-fry the aromatics: Add a drizzle of oil if the pan is dry. Toss in garlic and ginger. Cook 30 seconds until fragrant.
Add veggies: Stir in the onion and bell peppers.
Cook 3–4 minutes, tossing often, until crisp-tender. You want some bite left.
Add the pineapple: Stir in the pineapple chunks and cook 1–2 minutes to warm and lightly caramelize the edges.
Combine with chicken: Return the cooked chicken and any juices to the pan. Toss to combine.
Pour in the sauce: Give the sauce a quick stir, then pour it into the pan.
Cook 1–2 minutes, stirring, until the sauce thickens and coats everything.
Taste and adjust: Add more vinegar for tang, soy for salt, or sugar for sweetness. If it’s too thick, splash in a bit of water or pineapple juice.
Finish and serve: Remove from heat. Top with green onions and sesame seeds.
Serve hot over rice or noodles.