Prep the chicken. Pat the chicken dry and season lightly with salt and pepper. Cut into 1-inch pieces so they cook evenly.
Whisk the sauce. In a bowl, combine pineapple juice from the can (reserve the chunks), soy sauce, ketchup, rice vinegar, brown sugar or honey, garlic, ginger, sesame oil (if using), and red pepper flakes. Taste and adjust sweetness or tang as needed.
Brown the chicken. Heat the oil in a large skillet over medium-high.
Add the chicken in a single layer and cook 4–6 minutes, stirring once or twice, until lightly browned and just cooked through. Transfer to a plate.
Sauté the veggies. In the same pan, add the onion and bell pepper with a pinch of salt. Cook 3–4 minutes until slightly softened and a bit charred at the edges.
Combine with sauce. Return the chicken to the pan.
Pour in the sauce and add the pineapple chunks. Stir to coat and bring to a gentle simmer.
Thicken the glaze. Mix cornstarch with water to make a slurry. Stir it into the pan and simmer 2–3 minutes, until the sauce turns glossy and clings to the chicken.
If it gets too thick, splash in a bit of water.
Finish and taste. Adjust salt, pepper, or vinegar to balance sweetness and tang. Remove from heat.
Serve. Spoon over hot rice, then top with green onions and sesame seeds if you like.