1In 4-quart Dutch oven, cook beef and onion over medium-high heat 5 to 7 minutes, stirring occasionally, until beef is thoroughly cooked; drain.
2Mix beef mixture and remaining ingredients except buns in 3 1/2- to 4-quart slow cooker.
3Cover and cook on Low heat setting 7 to 9 hours.
4Stir well before serving. To serve, fill each bun with about 1/3 cup beef mixture. Beef mixture will hold on Low heat setting up to 2 hours; stir occasionally.