Cut chicken breast into bite-sized pieces about 1 to 1.5 inches. Pat dry with paper towels. Mince garlic, dice bell pepper, chop green onions. Drain pineapple if using canned, reserving juice. Have cooked rice ready. Mix cornstarch with 1 tablespoon water in small bowl.

Heat oil in large skillet or wok over medium-high heat until shimmering. Add chicken pieces in single layer without crowding. Cook undisturbed for 2 to 3 minutes until bottom is golden brown. Flip and cook 3 to 4 minutes more until cooked through to 165°F and no longer pink. Transfer to plate.

In same pan with browned bits, add minced garlic. Sauté for 30 seconds, stirring constantly, until fragrant and just starting to turn golden. Pour in soy sauce, pineapple juice, honey, and rice vinegar. Stir to combine and scrape up browned bits. Bring to simmer over medium heat.

Stir cornstarch slurry to recombine, then slowly pour into simmering sauce while stirring constantly. Continue cooking and stirring for 2 to 3 minutes until sauce is glossy, syrupy, and coats the back of a spoon.

Return cooked chicken to pan, stirring to coat with sauce. Add pineapple chunks and diced red bell pepper. Stir gently for 1 minute until pineapple is warmed and bell pepper softens slightly but retains crunch.

Reduce heat to medium-low. Add cooked rice to pan. Gently fold rice into sauce using spatula, breaking up clumps and ensuring every grain is coated. Cook 2 to 3 minutes, stirring occasionally, until rice is heated through and has absorbed some sauce.

Remove from heat. Taste and adjust seasoning if needed. Sprinkle chopped green onions and sesame seeds over top. Serve immediately while hot, with red pepper flakes on side for those who want heat.