Drain the pineapple, but save keep 1/4 cup of the juice from canned pineapple. Set the juice aside in a small bowl. Set the pineapple aside.
In the bowl with your pineapple juice, add ketchup, vinegar, brown sugar, and soy sauce. Whisk to combine. Set aside.
Cut onions and peppers into 1-inch pieces. Set aside. Cut chicken into 1-inch pieces. Sprinkle with 1 teaspoon of salt and cornstarch. Toss until well coated.
Mince garlic with a knife. Using a cheese grater, grate ginger.
Heat 1 tablespoon of the oil in a large nonstick frying pan over medium-high heat until shimmering. Add the onion and stir-fry for 1 minute. Add the peppers, season with 1/4 teaspoon of the salt, and stir-fry for about 3 to 4 minutes.
Add the garlic and ginger to the vegetables and stir-fry until fragrant, about 30 seconds. Transfer to a plate and set aside.
Heat the remaining 2 tablespoons oil in the pan over medium-high heat until shimmering. Add the chicken and spread into an even layer. Cook without stirring until browned on the bottom, 3 to 4 minutes. Flip and cook without stirring until the chicken is browned on the second side and cooked through, 3 to 4 minutes more.
Return the vegetables to the pan. Add the pineapple and sauce into the pan.
Stir-fry until thickened and coats the chicken and vegetables, 1 to 2 minutes. Serve over rice.
