Prepare the Chicken:In a bowl, season chicken with salt, pepper, and recommended garlic powder or soy sauce. Let it sit for 5–10 minutes while you prep the sauce and veggies.
Make the Sauce:In a small bowl, whisk together vinegar, sugar, ketchup, soy sauce, and red pepper flakes (if using). For a thicker sauce, mix in cornstarch. Set aside.
Cook the Chicken:Heat 2 tbsp vegetable oil in a large skillet over medium-high heat. Add chicken and cook for 5–7 minutes, until browned and cooked through. Add garlic in the last minute of cooking.
Add Veggies & Pineapple:Stir in chopped bell peppers, onion, and pineapple. Sauté for 2–3 minutes until just tender.
Combine & Simmer:Pour the sauce over everything. Stir to coat and let simmer for 5–8 minutes, until the sauce thickens and flavors meld.
Serve:Serve hot. Garnish with sesame seeds or green onions if desired.