Add 1/2 cup water to reserved 2 tablespoons ketchup mixture; whisk in flour until well blended and smooth. Place roasting pan over medium-high heat on the stovetop. Add beef broth; cook, stirring and scraping up any browned bits from bottom of pan, 2 minutes. Slowly whisk in flour mixture; cook, stirring, until thickened, about 3 minutes. Pour into a small serving bowl. Thinly slice roast. Serve with onions and gravy.