Heat 2 tablespoons oil in heavy pot over high heat, about 30 seconds. Add garlic; fry until brown. Add onions; stir fry 3 minutes. Remove onions and set aside.
Heat remaining oil in pot. Add chicken and curry powder. Stir fry 3 minutes. Add sugar, salt, soy sauce and water. Mix in potatoes (or you can add it carrots too). Bring mixture to a boil. Reduce heat and cover. Simmer 20 minutes.
Mix in onions. Cover and simmer 10 minutes longer. Stir cornstarch and water mixture into chicken. Boil 1 minute or until thick.