Cook beef and onion in 10-inch skillet over medium-high heat until beef is well browned, stirring to break up meat. Pour off fat.

Stir soup, milk and Worcestershire into skillet.Arrange potatoes on bottom of slow cooker liner.Spoon beef mixture over top of potatoes, then cover with cheese.

Cook on low for 6-8 hours or on high for 4 hours. Serve hot.