Combine ground beef, breadcrumbs, egg, finely chopped onion, salt, and black pepper in a bowl.
Shape the mixture into small, evenly sized meatballs.
Heat butter or oil in a skillet and brown the meatballs on all sides until cooked through. Remove and set aside.
In the same skillet, melt butter and stir in flour to create a smooth roux.
Gradually add beef broth, stirring continuously, then mix in heavy cream, a little Dijon mustard, and Worcestershire sauce.
Simmer until the sauce thickens, then return the meatballs to the skillet.
Cook for another 5 minutes, allowing the meatballs to absorb the creamy sauce. Serve warm with mashed potatoes or noodles.