Preheat the oven to 375°F (190°C) and lightly spray a 9x13-inch baking dish with cooking spray.
Make the teriyaki sauce: In a medium saucepan combine 1/3 cup soy sauce, 1/3 cup packed light brown sugar, minced garlic, ground ginger, and red pepper flakes; bring to a boil over medium-high heat, whisking frequently.
In a small bowl stir 2 tablespoons cornstarch into 1/2 cup water until dissolved.
Whisk the cornstarch mixture into the boiling soy mixture, reduce heat and simmer 1–2 minutes, stirring often, until the sauce thickens; remove from heat and set aside, reserving some for serving if desired.
Assemble the casserole: In the prepared baking dish spread 1 1/2 cups white minute rice, then add the canned pineapple with its juice, diced red bell pepper, and 1 1/4 cups low-sodium chicken broth. Stir gently to combine and, if using, add 1–2 tablespoons chili garlic sauce to taste.
Arrange the 1 pound thin-sliced chicken breasts evenly over the rice mixture.
Spoon a generous amount of the prepared teriyaki sauce over each piece of chicken, keeping the reserved sauce separate to avoid cross-contamination with raw chicken.
Cover the dish tightly with foil and bake for 30 minutes.
Remove the foil and bake an additional 5–10 minutes, until the chicken is cooked through and rice is tender.
Garnish with sliced green onions and optional sesame seeds, and serve immediately with the reserved teriyaki sauce if desired.