Preheat the oven to 400ºF. Cut the onion, bell peppers, and chicken breasts into one-inch pieces. Drain the pineapple well, reserving the juice for the sauce. Place the onion, bell pepper, chicken, and pineapple chunks on a large sheet pan in a single layer. Use two sheets, if needed, to prevent the chicken and vegetables from piling on top of each other. They need a little room to brown correctly.
Drizzle the cooking oil over the ingredients on the sheet pan, followed by the garlic powder, ground ginger, and a pinch or two of salt and pepper. Toss the chicken, bell peppers, onions, and pineapple until they are evenly coated in oil and spices.
Bake the chicken and vegetables in the oven for about 40 minutes, or until they are slightly browned on the edges. Stir half way through the baking time to redistribute the heat and allow excess moisture to evaporate.
While the chicken and vegetables are baking, prepare the sauce. In a small sauce pot whisk together the reserved pineapple juice (about 1 cup), ketchup, brown sugar, vinegar, soy sauce, and cornstarch until the cornstarch is fully dissolved. Heat the mixture over medium flame, stirring often, until it begins to simmer. When it begins to simmer the cornstarch will begin to gel and thicken the sauce. Once the sauce has thickened to a glaze, remove it from the heat and set it aside until ready to use.
When the chicken and vegetables have finished baking, remove them from the oven and pour the prepared sauce over top. Stir until everything is coated in sauce.
Serve the sweet and sour chicken over cooked rice with sliced green onions sprinkled over top.