Heat the oil in a large skillet over medium-high heat.
Add the chicken to the pan and cook for 5 to 6 minutes, stirring occasionally, until browned on all sides.
Add the minced garlic and ginger and cook for 1 more minute until fragrant.
Pour in the drained pineapple chunks, ½ cup reserved pineapple juice, soy sauce, brown sugar, and vinegar.
Stir well to combine all the ingredients in the pan.
Reduce the heat to medium and let the mixture simmer for 5 to 7 minutes, or until the chicken is fully cooked through.
In a small bowl, whisk together the cornstarch and cold water until smooth.
Slowly pour the cornstarch slurry into the pan, stirring continuously as the sauce thickens.
Let it cook for 2 to 3 more minutes until the sauce is glossy and coats the chicken well.
Remove from heat and garnish with chopped green onions, sesame seeds, or red pepper flakes, if using.
Serve hot over rice, noodles, or with steamed vegetables.