Slice onion into ½" slices, cut bell pepper into small chunks and cut pineapple into chunks.
For the sauce add ketchup, brown sugar, rice vinegar, water, soy sauce and cornstarch to bowl and mix until combined.
Cut chicken into cubes and add cornstarch and salt and mix well.
Add 2 tbsp oil to hot skillet, add chicken and on medium heat sear chicken until browned on both sides.
Remove from skillet, add 2 tbsp more of oil and sauté onions for about 3 minutes or until slightly softened and browned.
Add bell pepper and sauté for 3 minutes until softened, then add pineapple and sauce and cook just until sauce starts to thicken. Add chicken and cook another minute or until chicken is fully cooked.
Garnish with chopped green onions and sesame seeds. Serve with rice or noodles.