Preheat the oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes. Place the peppers upright in a baking dish.
Heat the vegetable oil in a large skillet over medium-high heat.
Add the cubed chicken, season lightly with salt and pepper, and cook for 4 to 5 minutes until lightly golden and just cooked through.
Reduce the heat to medium. Pour in the teriyaki sauce and pineapple juice, stirring well to coat the chicken.
In a small bowl, mix the cornstarch with 1 tablespoon cold water to make a slurry. Stir it into the skillet and cook for 1 to 2 minutes until the sauce thickens and looks glossy.
Remove the skillet from the heat. Stir in the cooked rice, diced pineapple, and half of the chopped green onions.
Spoon the mixture evenly into the prepared peppers, pressing gently so each one is well filled. Top with the remaining green onions.
Add about 1/4 cup water to the bottom of the baking dish. Cover loosely with foil.
Bake for 25 to 30 minutes, or until the peppers are tender.
Remove from the oven, uncover, and garnish with sesame seeds if desired.
Serve these Teriyaki Pineapple Chicken Stuffed Peppers warm.