Pat the chicken breast dry with paper towels, season lightly with salt and black pepper, and cut into bite-sized pieces for even cooking.
Heat the vegetable oil in a large skillet over medium high heat, add the seasoned chicken in a single layer, and cook, stirring occasionally, until golden and cooked through; transfer the chicken to a plate and set aside.
In the same skillet, add the chopped onion and red bell pepper, cooking until softened and lightly browned, then stir in the minced garlic and ginger just until fragrant.
Pour in the reserved pineapple juice, soy sauce, honey, and vinegar, scraping the bottom of the pan to release any browned bits, and bring the mixture to a gentle simmer.
Whisk the cornstarch with the cold water in a small bowl to make a slurry, then slowly drizzle it into the simmering sauce while stirring until the sauce thickens and turns glossy.
Add the pineapple chunks and the cooked chicken back into the skillet, tossing well so everything is coated in the sauce, and let it simmer together for 2 to 3 minutes to heat through.
Spoon the warm Pineapple Chicken mixture over bowls of hot cooked rice, then garnish with sliced green onions, sesame seeds, or red pepper flakes as desired.