Whisk pineapple juice, soy sauce, brown sugar, rice vinegar, garlic, ginger, and sesame oil in a bowl.
Marinate the chicken with half the sauce in a zip-top bag. Chill 15–20 minutes.
Heat vegetable oil in a large skillet over medium-high heat.
Sear the chicken until browned and cooked through, 4–5 minutes per side. Transfer to a plate.
Simmer the reserved sauce in a small saucepan. Stir in cornstarch mixed with water and cook until glossy, 1–2 minutes.
Stir in pineapple chunks. Pour the thickened sauce over the chicken in the skillet and toss. Simmer 2 minutes.
Finish with green onions, sesame seeds, and red pepper flakes if using. Serve hot over steamed rice.