Preheat the oven to 425 F and line a large sheet pan with parchment or foil.
Whisk the soy sauce, pineapple juice, brown sugar, olive oil, garlic, ginger, cornstarch, and red pepper flakes in a bowl.
Add the chicken to a large bowl, season lightly with salt and black pepper, then toss with half the sauce.
Spread the chicken, pineapple, peppers, and onion onto the sheet pan in a single layer.
Drizzle with the remaining sauce and roast for 20 to 25 minutes, stirring once halfway through.
Cook until the chicken is done and the vegetables are tender and lightly caramelized.
Serve hot over rice and spoon any extra sauce from the pan on top.