1Cook beef in 10-inch skillet over medium-high heat about 8 minutes, stirring frequently, until brown; drain. Stir in ketchup, onion and mustard.
2Spray 3 1/2- to 4-quart slow cooker with cooking spray. Spoon beef mixture into cooker. Top with cheese.
3Cover and cook on low heat setting 3 to 4 hours or until mixture is hot and cheese is melted.
4Serve beef mixture with buns. Beef mixture will hold on low heat setting up to 3 hours.