Place meat and cornstarch in a ziplock bag and shake until chicken is evenly coated.
Heat oil in a large non-stick skillet over medium-high heat. Add chicken and brown on all sides, about 2-3 minutes.
Spray the bottom of slow-cooker with cooking spray and then place chicken, onion, and sauce in. Stir until all the chicken is coated in sauce. Cook on low for 1 1/2 hours and then throw in the pineapple and bell peppers and cook for another 1 1/2 hours until peppers are tender.