In a large skillet or wok, stir-fry red pepper and onion in oil for 3-4 minutes or until crisp-tender. Drain pineapple, reserving the juice in a 2-cup measuring cup; set pineapple aside. Add enough water to the juice to measure 1-1/3 cups; stir in the vinegar, soy sauce and ketchup.
In a large bowl, combine brown sugar and cornstarch. Stir in pineapple juice mixture until smooth. Gradually add to the skillet. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in pineapple and heat through.
Meanwhile, cook chicken in oven according to package directions. Stir into pineapple mixture. Serve immediately.