Brown the beef and onion in a skillet for 9 minutes until the beef reaches 160°F and drain the fat.
Melt the butter, whisk in the flour for 1 minute, then gradually whisk in the milk, mustard, Worcestershire, salt and pepper until slightly thick.
Layer half the potatoes, half the beef, 1 cup cheddar and half the sauce in a buttered 6-quart crockpot and repeat, reserving 1 cup cheddar.
Cover and cook on low for 5 1/2 to 6 hours until a knife slides through the center potatoes.
Scatter the remaining cheddar over the top and cook uncovered on high for 15 minutes until melted.