1. Cook ground round and onion in a large nonstick skillet over medium-high heat until meat is browned, stirring until meat crumbles. Drain and return to skillet.
2. Stir ketchup and next 5 ingredients into meat mixture; bring to a boil. Cover, reduce heat, and simmer 10 minutes, stirring often. Spoon mixture evenly over bottom halves of buns; cover with bun tops.
Note: How do you know if you're buying the kind of ground beef with the least amount of fat? First, look for the cut of meat on the label. Ground chuck is higher in fat; ground round and sirloin are leaner. Or look for the percent lean; for example, "95 percent lean ground beef." This means 95 percent lean and 5 percent fat and indicates the type of ground beef recommended for heart-healthy cooking.