In a skillet over medium heat, brown the ground beef with the onion, until the meat is no longer pink, and the onion is soft; drain fat.
In a Dutch oven or large saucepan, combine the ground beef mixture, and pureed tomatoes. Season with Worcestershire sauce, chili powder, salt and pepper. I just keep adding stuff until it tastes good. Bring to a boil, and let simmer for 15 to 20 minutes to blend flavors. Serve over rice.