Season chicken breast with salt and pepper. Sprinkle cornstarch over chicken, tossing to coat.

Heat vegetable oil in a nonstick skillet over medium-high heat. Add chicken pieces in a single layer and cook for 5–7 minutes until golden brown.

In a medium bowl, whisk together pineapple juice, brown sugar, soy sauce, rice vinegar, garlic, and ginger until brown sugar is dissolved.

Pour sauce mixture into the skillet, scraping up any brown bits. Bring to a simmer for 2–3 minutes.

Mix cornstarch with water to create a slurry, whisk into the bubbling sauce and cook for 1–2 minutes until thickened.

Return cooked chicken and add pineapple chunks. Stir to coat and cook for another 2-3 minutes.

Garnish with green onions if desired, and serve over rice or noodles.