Heat oil in 12-inch nonstick skillet over medium-high heat and cook onion, stirring occasionally, 2 minutes or until onion is tender. Stir in garlic and cook 30 seconds.
Stir in water, Knorr® Homestyle Stock - Chicken, ketchup, brown sugar and vinegar. Bring to a boil over high heat. Reduce heat to low and simmer, stirring occasionally, 5 minutes. Stir in chicken and heat through.
Serve, if desired, on toasted hamburger buns.