In a small bowl, whisk together the reserved ½ cup pineapple juice, ketchup, vinegar, coconut aminos, garlic powder, and ginger. Slowly whisk in the cornstarch until smooth; set aside.
In a medium bowl, toss the chicken pieces with salt and black pepper to coat evenly.
Heat a large skillet over medium-high heat. Add 1 teaspoon of oil to the pan, then reduce the heat slightly. Add the chicken and cook without stirring for 2–3 minutes. Once browned, flip and continue cooking until almost cooked through. Remove to a plate.
Add the remaining 2 teaspoons of oil to the skillet. Toss in the onion and carrots, cooking for about 4 minutes while stirring often. Then, add the bell peppers and cook for another 4–5 minutes until crisp-tender.
Return the chicken to the skillet along with the drained pineapple chunks. Stir the sauce mixture one more time, then pour it into the skillet. Cook, stirring frequently, until the sauce thickens and evenly coats the chicken and veggies—about 2–3 minutes.
Remove from heat and garnish with green onions and sesame seeds, if desired. Serve immediately.