Brown ground beef in large skillet over medium-high heat until thoroughly cooked, stirring frequently to break into small pieces. Drain. Stir in 2 tablespoons ketchup, onion and 1 teaspoon mustard.
Spray 3 1/2 to 4-quart slow cooker with nonstick cooking spray. Spoon beef mixture into sprayed slow cooker. Top with cheese.
Cover; cook on low setting for 3 to 4 hours. Just before serving, stir beef mixture. Spoon 1 rounded tablespoon mixture into each bun. Serve with desired toppings.