Combine your potatoes and onions and place them in a 6 quart slow cooker.
In a medium saucepan over medium heat, add 3 tablespoons of your butter.
Once the butter has melted add the flour, mustard, Worcestershire sauce and pepper and stir until combined.
Add the milk to the saucepan and stir frequently until it comes to a boil.
Reduce heat to medium-low, stirring frequently, and cook the mixture until thick and bubbly.
Add the Cheddar cheese and stir until the cheese melts.
Pour the cheese mixture over your potatoes and toss to coat.
Cover and cook on low for 5 1/2 to 6 1/2 hours or on high for 2 1/2 to 3 1/2 hours, or until the potatoes are tender.
Melt your remaining 2 tablespoons of butter and combine with your Italian bread crumbs. Sprinkle the bread crumbs evenly over your potatoes.
Cook uncovered for 30 minutes.