Whisk soy sauce, pineapple juice, ketchup, rice vinegar, brown sugar, garlic, ginger, red pepper flakes, and sesame oil.
Stir cornstarch with water to make a smooth slurry. Set aside.
Heat neutral oil in a large skillet over medium-high heat.
Season chicken with salt and black pepper. Sear until browned and cooked through, 5 to 7 minutes. Transfer to a plate.
Sauté onion and bell pepper in the same skillet until crisp-tender, about 3 to 4 minutes.
Return chicken to the pan. Add pineapple chunks and pour in the sauce. Bring to a simmer.
Drizzle in the cornstarch slurry while stirring. Cook until thick and glossy, 1 to 2 minutes.
Finish with green onions and sesame seeds. Serve hot over rice or noodles.